How is lift generated due to Coanda effect? I can understand the generation of lift via Newton's 3rd law but I cannot understand via Coanda effect. I watched a video on YouTube that said that for the upper side of the aerofoil the pressure will be lower near the surface because the fluid has to 'stick' on the surface and they said that on the lower side the pressure must be higher for the same reason. But how can the pressure be higher on the lower side due to the same reason. Also fluid sticks on the surface due to viscosity so why the Coanda effect ?? Please check this link. 
m.youtube.com/watch?v=w78JT6azrZU
 A: The operative idea  here seems to be the exploitation of convex, or at least  "tightly" curved surfaces, especially with regard to aircraft wing sections. From reading various articles, it seems to be quite easy to misjudge if and when the Coandă effect occurs, even to the extent of confusing the Bernoulli effect with the Coandă effect.

Image source and Text Extract Thermofluids UK:

The  Coandă effect is the phenomena in which a jet flow attaches itself to a nearby surface and remains attached even when the surface curves away from the initial jet direction. In free surroundings, a jet of fluid entrains and mixes with its surroundings as it flows away from a nozzle.
When a surface is brought close to the jet, this restricts the entrainment in that region. As flow accelerates to try balance the momentum transfer, a pressure difference across the jet results and the jet is deflected closer to the surface - eventually attaching to it.
Even if the surface is curved away from the initial direction, the jet tends to remain attached. This effect can be used to change the jet direction. In doing so, the rate at which the jet mixes is often significantly increased compared with that of an equivalent free jet.


Quite a few aircraft have been built with extra engines specifically designed to direct flow over the top of the wing, where the camber is most pronounced. Air directed  over the wing can be "bent down" towards the ground using flaps and a jet sheet blowing over the curved surface of the top of the wing for a, usually temporary, high lift effect. This may be particularly applicable in regions where mountainous terrain restricts runway length.
The Coandă effect augments the normal role of flaps by significantly increasing the velocity gradient in the shear flow in the boundary layer over the uper surfae of the wing.  In this velocity gradient, particles are blown away from the surface, thus lowering the pressure in that region.
According to Wikipedia, the Coandă effect is often  misapplied to situations where it is not the cause, such as the well known "trick" of holding the curved side of a spoon close to a running tap, where surface tension is the cause of the pull into the stream of water.
If you look at your illustration above, because of the particular profile of the wing section, the lower surface is far less curved than than the upper one. If you look out of the windows of any modern aircraft as it extends its flaps, the downwards curve is obvious and this supports the idea mentioned above of fitting blown air devices to exploit this convex surface.

But how can the pressure be higher on the lower side due to the same reason. Also fluid sticks on the surface due to viscosity so why the Coanda effect.

I think the illustration in your post above,  or possibly the intreperation of it, may be incorrect. The bottom surface is obviously not where the greatest camber appears, so pressure differences will occur.
From Coandă Effect Wikipedia

To compare experience with a calculation we refer to a two-dimensional plane wall jet of width h along a circular wall of radius r. A wall jet follows a flat horizontal wall, say of infinite radius, or better whose radius is that of the Earth, without separation because the surface pressure as well as the external pressure in the mixing zone is everywhere equal to the atmospheric pressure and the boundary layer does not separate from the wall.
With a much smaller radius (12 centimeters in the image) a transverse difference arises between external and wall surface pressure, creating a pressure field depending upon h/r, the relative curvature. This pressure field can appear between a zone around and after the origin of the jet where it gradually arises, and a zone before the point where the jet boundary layer separates at atmospheric pressure where it gradually decreases.


Image Source: Marcel kadosch - Own work.
Surface pressure of a wall jet along a circular wall

Experiments made in 1956 at a Reynolds number 106 and various widths h show the pressure measured along a circular wall, entered at a horizontal distance from the origin of the jet.
Above a critical h/r ratio of 0.5 only a local effect formed by these two zones each extending over a small angle 9° is observed. This is not a Coandă effect. If the h/r ratio is smaller than the critical value 0.5, an additional deflection that can validly be called a true Coandă effect occurs in between at a nearly constant pressure, as in a conventional wall jet.

Finally, the NOTAR (no tail rotor) equipped  helicopters used the effect to replace the usual shaft driven tail rotor.

Obviously, the word "hug", in the illustration is a reference to the effect described at the start of this answer.
A: If the question is :Lift of an airfoil and Coanda effect, the answer is implicit in my contribution to: Wikipedia the free encyclopedia: Coanda Effect: Conditions for existence. 
Coanda Effect has nothing to do with airfoil lift, it is a purely inertial effect thoroughly explained by an inviscid irrotational theory involving  no vortex at all, bounded or not. The boundary layer  of this inviscid flow and the turbulent mixing with ambient air are the only places for viscosity effects which do not produce any Coanda Effect: on the contrary  their influence is to put a limit and finally impede the production of a Coanda Effect. Marcel Kadosch 
A: 
As shown in the figure, we define that the curvature center direction of the streamline is the interior of the streamline and the opposite direction is the exterior of the streamline. Looking at the streamline at the bottom of the wing, we can see that the wing is outside the streamline. Therefore, the pressure of the airflow increases because the airflow is changed by the compression of the wing. Looking at the streamline at the top of the wing, we can see that the wing is inside the streamline. Wings are pulled to change the direction of the flow, so the pressure of the flow decreases. So the wing has a pressure difference, so the wing can get lift.

Therefore, the lift of the wing is not generated by Coandeffector, because Coandeffector can not be without viscosity, and the lift of the wing can be generated without viscosity.
A: If you can understand the generation of lift via Newton's 3rd law; then applying the  Coanda effect is simple. 
Just to be clear, the Newtonian explanation of flight: The airplane flies through a mass of air ('m') that the wings accelerate ('a') downwards, to create a downward force (Force = ma). The 'equal & opposite' upward force generates lift. https://youtu.be/GAAOYOmtEQI 
Demonstrate the Coanda effect by an example: If an airplane's flat wings are replaced with curved wings. The Coanda effect on the top-side of the curved wings, increases the mass of air ('m') accelerated downwards; this then increases the downward force (F = ma).  In turn, this increases the 'equal & opposite' upward force (lift). 
A: Most of the lift on the top side of the airfoil is caused because of the decrease of atmospheric pressure (over 1000 lbs. per. square foot) pushed away by incoming air, at an angle of attack, from the bottom of the wing.
The top of the wing creates appx. 67% of the lift!
