How to downgrade your iPhone from 3.1.3 to 3.1.2

First Checkout the Version by following the Steps Below

Turn on the IPhone and follow the Steps Below

1. Go to Settings

2. General

3. About.

If the Version is 3.1.3 then follow the steps below

1. Go to the link below and download the ipsw file for 3.1.2 model

http://www.felixbruns.de/iPod/firmware/

2. Put your iPhone 3G in DFU mode with following instructions.

a. Connect your iPhone to your computer.

b. Turn iPhone off.

c. Hold power and home together for 10 seconds (exactly).

d. Release power but keep holding home until the computers beeps (observed on a PC) as a USB device is recognized.

e. A few seconds later iTunes should detect your iPhone. If the Restore Logo is present on the screen, you are in Recovery Mode, not DFU.

There will be no display on the IPhone Screen and a pop will appear on the screen as the one below.

Browse the ipsw file and thats it go and have a cofee and by that time IPhone would have downgraded to 3.1.2

if you encountered an error message like 1011, 1013, 1015, just ignore them(Actually the firmware 3.1.2 is already successfully installed)

Note: Sometimes the IPhone may loop through recovery mode every-time you start the IPhone in that case please refer the blog " How to Fix Re-looping after Restore or Jailbreak"

Likewise you can downgrade and upgrade different versions, but its not going to downgrade the baseband version.