Help Undo OS upgrade through Kies


I few weeks ago I decided to upgrade the OS on my Galaxy S2 to the latest version Kies would let me upgrade to. I did this to unlock some features in apps that were available for the updated OS.

However, I have noticed that my phone has slowed down a lot and heats up a lot too. I would like to go back to the original OS after creating backup of all photos/videos/contacts etc.

Can someone suggest the best way of doing so.