How to downgrade ios 6 to ios 4.1?

My iphone 3gs has been lagging since I updated to ios 6. My camera shutter won't open anymore and sometimes it just randomly crashes, closes apps, or freezes. I heard it's because the 3gs wasn't meant to have ios 6 and the best software for my phone is 4.1. Does anybody know how to downgrade the software? I can only figure out how to upgrade it and I don't want that at all.

If you backed it up like you should have, you can just restore the back up.