UPDATE : There seems to be a bug in ultrasn0w 1.2.8 when used with iPhone 4 having modem 01.59.00 on iOS 6.1. So Pls. do not proceed with following steps until this gets fixed.

Users relying on Ultrasn0w software carrier unlock can now upgrade to Apple iOS 6.1 as it can now be jailbroken using evasi0n and is now supported by ultrasn0w 1.2.8 update, do keep in mind that you must have the baseband (modem) preserved to one supported by ultrasn0w. Follow the guide posted next to upgrade, jailbreak and unlock iOS 6.1 while preserving the baseband.