If you are on At&T there are only two ways. One, you have to have downloaded NetShare when it was in the App Store. Or two, you have to be running 3.0 or 3.0.1 and use the help.benm.at hack. However, if you did not get your SHSH blobs saved via Cydia you cannot downgrade to 3.0 or 3.0.1. Other then jailbreaking I know of no other way to accomplish this.