upgrade iphone 3g OS to ios 4.1

[kmurali70]

I have a iphone 3g with version (3.1.2 (7D11))
and BB 5.12.1
I would like to upgrade to ios 4.x
Which ios should I upgrade to, if i need to JB and unlock, I use a prepaid tmobile SIM card.

which is the most reliable guide to do thi and which ios should I upgrade to. ios 4.1, 4.2 or 4.1.3 as some say.

My 3g is JB and unlocked as of now and would like to have a smooth upgrade/JB/unlock, since the last time I had a tough time to do the same.
Thanks

[iYeow]

If you don't have 4.1 shsh blob saved, use ifaith to save your 4.1 blob first.
Now , for PC : download snowbreeze 2.2.1 and download the stock 4.2.1 firmware and build your custom firmware of 4.2.1, use the pwned dfu and restore thru itunes to cfw of 4.2.1
Once restored, you can unlock again your preserved baseband by Ultrasn0w from Cydia