Updating 3.0 - 3.1.3

[86b]

Hi,

i'm new to iphones. my gf has an iphone 3gs 3.0 and she has updated an app, but it requires 3.1+ firmware to use. how do i update it to 3.1.3 without updating to 4.0.2 then downgrading it to 3.1.3? her phone is not jailbroken or unlocked.

i've done some research, can i just download 3.1.3 firmware and restore it on the phone? is it that simple?

thankyou

Bump

[Will01]

No you can't just flash 3.1.3. If she has never been JB she wont have SHSH saved, meaning you can't restore to 3.1.3. 4.0.2 is your only option.