iPhone 3G upgrade from 4.0 to 4.1 preserving baseband?

[Faux Carnival]

Hello, I have an iPhone 3G with baseband 05.13.04. I need to upgrade from 4.0 to 4.1 preserving my baseband. I'm on Windows. What do I need to do?

Thanks.

 

[PsstGreek]

To go to 4.1 you would have to have the SHSH saved in Cydia. You'll also need a custom ipsw or create one in PwnageTool.

 

[Faux Carnival]

What does a saved SHSH mean? And can I find this so called custom firmware on the internet? Thanks.