Need Help upgrading iphone 3gs from 3.1.3 to ios 4

Hello peeps,

I've been using my Iphone 3gs for a while now on IOS 3.1.3 and would really love to upgrade it. The modem firmware version is 05.12.01.

I'll greatly appreciate it if someone can point me in the right direction.

Thanks in advance.

Please don't double post, I just finished answering your same question in another forum.

Short answer, if you don't have a 4.x SHSH, you can't upgrade to 4, you can only go to 5. Please check your other thread for further information.