How transformer convert into ac genarator?

It does not in a true transformer.

If you are talking about items in a charger or a power supply for game, phone, laptop and so on it has three basic stages.

1. A set of coils wound inside each other step the voltage down to the target voltage. This can also be done using capacitors and whizy electronics. The whizy electronics can skip the next step (2) By switching internally and feeding the capacitors very brief full AC voltage.
2. Then the resulting power goes through a rectifier becoming almost DC power. The problem at this stage is that the AC to DC rectifier leaves "Ripples" in the power that can cause serious trouble for DC run electronics.
3. It will go into a Capacitor "block" (My term) to smooth the ripples out.

This is a seriously simplified version, but this is the basic steps.