How can I downgrade my HTC Explorer from Android 4.0 to 2.3 Android 4.0 is installed on my HTC Explorer, but now I want to install again the previous version which was 2.3 Gingerbread.

Could anyone please tell me how to do that?
One option is using RUUs. A few Google searches should get you going.

Another is flashable stock ROM zips. A few searches in the XDA Developers forum for that device should put you in track.