Use below tutorial to update to iOS 6, 6.0.1 without problems.

You need to follow till Step 9. One your phone is jailbroken and baseband preserved. After that use this Ultrans0w fixer guide to unlock your phone. 

This tutorial might also help. Use simle mode in Sn0wbreeze.

One of our readers used this method. 

1. installed ios4

2. restored ios6.0.1 with no_bb option

3. i jailbroken it with redsnow 15b3 (just used ios6.0 ipsw because ios6.0.1 ipsw is not recognized) and installed cydia (this doesnt modify the bb)

4. in cydia i installed ultrasn0w fixer and ultrasn0w

5. restart device 

6. insert sim - OK - it works