Are you talking about a phone or a tablet? A factory reset should be all you need to do. Unless your device won't boot up at all, in which case you might need to enter recovery mode. Unfortunately, I only know how to do that on my own phone. If your device is rooted, you can download and install many different ROMs (OS's essentially).
Check out
www.xdadevelopers.com to find more information on any android device than you even need to know
