Question

When we give energy to electron then electron excit and go to other high energy shell due to gain energy but if electron goes to another shell then atom will unstable what is happen actually

Answer 1

When energy is provided to electron, the electron is excited to high energy level. But it is wrong to say that atom will be unstable. It will obviously be less stable than when the electron was at the ground state.
The most stable state is when the electron is at the ground level and hence it has least energy. The stability decreases when energy is provided and electron is excited to higher energy orbital.