Downgrade from ios 5 to ios 4.2.1 (Untethered)

Things need to be prepared :

1. Tiny Umbrella 5.00.12
2. GreenPois0n 1.0 RC6.1
3. fixrecovery-osx for 4.2.1
4. ios 4.2.1 ipsw

Follow this step :

1. Make sure that you have backup SHSHs before
2. Open TinyUmbrella, and start TSS Server
3. Make sure that you see SHSHs for ios 4.2.1 in List “All saved SHSHs”
4. Enter DFU Mode
5. Start iTunes 10.1
6. Restore ios 4.2.1 by pressing alt button and click Restore button
7. Find and Select ios 4.2.1 ipsw
8. Wait until iTunes finish the process
9. Maybe there will be an error
10. Enter DFU mode
11. Run FixRecovery
12. Wait until iPhone successfully Reboot
13. run GreenPois0n
14. Done

Good Luck