how do i update my iphone from 3.1.2 to 4.0?

[ShawnBaker]

it always gives me an error and i suspect it is because i dont have 3.1.3 but it doesnt give me the option to get 3.1.3 only 4.0
what is dfu mode and how do i put it in it

 

[sluggy]

I upgraded and my phone keeps turning itself off, I dunno if its the software or a fault on my phone

 

[L0g1c5]

Have you upgraded itunes to 9.2?

if not then u need to upgrade