Is it possible to downgrade from 6.1.3 to 5.1.1?

I have had 6.1.3 for a day and I can not stand it. It is horrible, terrible lag, none of my apps can stay open for longer than 5 minutes, maps are terrible, I can not even load the settings app, the most important application for the entire itouch. Also to change the brightness you have to tap into another tab which is annoying. How can I downgrade back to 5.1.1?