Hi guys,

I would like to share my experience on this topic. It took me quite a while to get all steps right, because there are not many useful instructions around the web.

FYI: This manual only describes how to upgrade to 4.1. What I head so far is 4.2.1 still risky to go for. While there are no major improvements for the iPhone 3GS anyways, I decided to go for 4.1, instead of eventually brick my phone!

!! NOTICE, YOU DO THIS AT YOUR OWN RISK !!

1) Backup your data before updating. (backup pic's manually, just in case they don't get thru).
2) Connect your iPhone and open TinyUmbrella 4.21.01 to store your SHSH keys.
-> Double check the log page if your required 4.1 shsh key is stored locally.
3) Open Pwnage Tool 4.1.2:
- Create a custom ipsw with Pwnage tool using the original apple ipsw version 4.1 for iPhone 3GS.
- Use "expert mode". Browse for apples 4.1 ipsw and hit next.
-> If you can't re-activate your phone thru iTunes, select "Activate the phone" in the "General settings" tab. (recommend)
- Let Pwnage tool create your 4.1 custom restore file.
- Afterwards put your iPhone in DFU mode with the help of pwnage tool.
4) Make sure iTunes is closed and hit the "Start TSS Server" button in your TinyUmbrella app.
5) Restore using iTunes 9.0.1.8. (www.oldapps.com) -> Hold down option key and select your custom ipsw.

DONE

If you have a different carrier than your phone is registered to, install yellowsn0w to unlock your phone.

Good luck

ps. I can't provide any more support. This should get you thru the whole procedure. If something unexpected happens, ask google for help.