UnLock Flash can help you flash the OS of Galaxy S, S II, SIII, Galaxy Sand Galaxy Note II to any versions you want. UnlockFlash is a free software. It is much simpler than Odin and KIES. What you need to do is select the update file and make several clicks.
You should backup all the apps, contacts, messages and other important data on your phone before flash, because this tool would wipe all these data on your phone.