Base on a superficial understanding of an Activity in Android (sounds like a window element), you just need to call Close
on the current window, then the previous one will then move to the foreground.
For future reference however, I would advise you not too create such analogies between two radically different technologies since the life cycle of an Activity in Android is radically different from a form in a Windows Form application. You should try to learn the framework rather than replicating an implementation from a different framework/platform.