If the midpoint between $+q$ and $-q$ has zero electric potential, in which part/region is the electric potential increasing and decreasing?
Potential increases in the direction which points opposite to the net electric field.
Take example of a $+ve$ charge. Potential is defined as the work done by external force per unit charge.
So if I bring a +ve unit charge (called test charge) towards the reference $+ve$ charge, I will have to do work against the electric force that the reference charge is applying on the test charge.
Consequently, the value of potential will be higher in the vicinity of the reference charge than at far away points because the work done by us is used up in overcoming the electric repulsions and stored up as potential energy.
So, potential increases in the direction opposite to electric field.