You can re-set the phone to factory settings, but I believe that all the updates will automatically download. I did a factory re-set and still have the newest update (Ice Cream Sandwich). The re-set did help with some of the files issues I was having, but the phone still changes things at random intervals. It also will sometimes show allmy stuff, sometimes ZERO of my files. There is no rhyme or reason to the stoppages.
You CAN "root" your phone and then put whatever system you want into it, but that requires quite a bit of technical knowledge and voids any warranty you have.