How to Downgrade iPhone 3GS from 3.1.3 to 3.1.2
So I'm sure many people on the forums have updated to 3.1.3 "accidentally", so I'm going to explain how I downgraded to 3.1.2 from 3.1.3.
First you must have had 3.1.2 jailbroken and you must have allowed cydia to save your ECID previously. Most people probably have.
Right so now you need to trick iTunes into verifying your restore of 3.1.2 with a fake server. Go to C://Windows/system32/drivers/etc and right click host and open with notepad. Now at the bottom of the file add this line.
Click file save, then close the window. You might not be able to select save, in this case you will need to right click the file and run as administrator.
Now download the 3.1.2 Restore file for your iPhone. I recommend downloading from somewhere such as thepiratebay as it will be faster, here is what you should be looking for depending on your iPhone. Make sure you download an .ispw file.
iPhone 3GS: iPhone 2,1 3.1.2
iPhone 3G: iPhone 1,2 3.1.2
Alright now that you have the restore, you must put your iPhone into recovery mode and restore to this file.
There are many complicated ways of doing this, to save you trouble here is the easiest way you'll ever find. Hold the home button and plug your iPhone into the computer. Keep holding the home button until you see a connect to iTunes picture on your phone.
Okay so your almost, almost done. Open iTunes and you should see a pop up saying that the iPhone must be restored. Click ok and hold shift on your keyboard and click the restore button in iTunes. Now an explorer window will come up, locate the 3.1.2.ispw file you downloaded earlier and select ok. Your iPhone will start restoring to 3.1.2.
Oh Noez I got error 1015! Ok all is good, now download irecovery from this link, iHackintosh iRecovery Package for Windows & Mac.rar
Now in the first folder you will see an installer for libusb, ignore everything else and install this on your computer, you will need it to use irecovery. Now download the second and extract it also, you should get some files, copy them all to your desktop. If your on windows 7 or Vista right click irecovery.exe and open properties and choose the compatibility tab, now select windows xp sp2.
Now open the start menu on your computer and choose all programs, accessories, command prompt. Type,
now type It will display "]" type the following things and press enter after each.
Now unplug your iPhone and hold the power and home button for 10 seconds, or until it turns off. Now hold the power button until you see the apple logo. Congratulations your now on 3.1.2! Plug into iTunes and activate!
setenv auto-boot true
If you need to jailbreak 3.1.2 check my guide: