I'm unable to figure out how. I've installed CCToggles first and couldn't find an option to disable 3G and go to 2G. I then installed CCSettings and that doesn't have an option either. Before jailbreaking, a few weeks back I updated my carrier settings and since lost the ability to go back to EDGE/2G. The only option available now is toggle between LTE on and off. I'm on iOS 7.0.6 and have just jailbroken using evasi0n.