how to upgrade iphone 3G os
hello, i'm selling my iphone 3G and i'm on 3.1.2 and i want to upgrade it to 3.1.3 before i sell it, at the moment it offers me the 4.? update and i don't want that. I have found and downloaded the 3.1.3 update but i don't know how to go about installing it.
any help would be great,