I don't understand how a photon energy get absorbed or emitted by a electron. when photon incident on electron it absorbs photon energy and go to excited state, and when it come down it emits energy. what exactly happens?
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I think to appreciate what "exactly happens", or how it happens in terms of more refined models, you'd have to embark on extended studies of atomic physics and/or quantum optics. But to the point:
What you are probably referring to is (stimulated) absorption and spontaneous/stimulated emission of radiation from an atom. In the most common case you'll have a so-called electrical dipole transition (also other kinds of transitions are possible, but usually far less probable). The charge distribution of the shell electron of interest makes up for an oscillating electrical dipole. For that it is usually required that the electron is in a (coherent) superposition of the lower and upper state of the transition (ground and excited state). If the electron is in an energy eigenstate (i.e. either the excited or the ground state) there won't be an oscillating electric dipole moment associated with the atom (accelerating charge is what causes radiation in the end), the charge distribution is stationary (the electron just sits there snapped around the positive rest of the atom in a kind of standing wave (pictorially speaking)). So under these conditions normally the atom would just sit there forever without ever emitting or absorbing light.
Now we can consider two cases: (A) absorption: a passing by quantum of light corresponds to a little jiggling in the electric field where the atom sits, if it's "resonant" with the atomic transition, oscillating right at the rate for making the electric dipole moment of the atom swing up and bring the atom from the ground to the excited state. If you have some physics background: Try to write down the expectation value of the atoms electric dipole moment once it is in a superposition of the ground and first excited state (for the hydrogen atom for example). You'll find that it oscillates in time at the "transition frequency".
(B) emission: here a passing-by light wave can cause the atom to have its electric dipole moment swing with the wave and de-excite, emitting a chunk of light of the very same kind as the passing-by light wave (that's called stimulated emission). Or there is another option: according to quantum electrodynamics, the mere jiggling of the electric fields in empty space throughout the atom is sufficient to excite the atom to have a bit of a swinging dipole-moment develop, at some point the atom will have returned to the ground state and have emitted the light wave chunk in the process (that's called spontaneous emission, and every physicist gets excited about it, as it is an effect seemingly caused by the miraculous vacuum :))
So you can really imagine this a bit like the hen/egg problem. It's all about the interplay between the oscillating electric field of the photon (light is an electromagnetic wave) and the atom possessing an oscillating electric dipole moment, when it is in a superposition of the ground and excited state. I deliberately changed pictures between particle/wave, discrete/continuous here: That's the challenge in imagination that quantum physics confronts us with at the end of the day.