Now, after upgrading to the newly release firmware by Apple iOS 4
You got some bugs and some errors, A lot of users asked me this
question "How to Downgrade iOS 4 to 3.1.3" So Answering my readers,
this guide will help you to Downgrade iOS 4 to iPhone 3.1.3 or 3.1.2
Steps:
Step 1: Download iPhone 3.1.3 / iPhone 3.1.2 Original Firmwares from Here.
Step 2: Put your Device in DFU Mode by following the guide posted here.
Step 3: This step is for iPhone 3GS, iPod touch 3G and iPod touch 2G (MC model) Owners who saved SHSH on Cydia for the Firmware you downgraded to, If you are on iPhone 3G or iPod Touch 2G then Skip this step.

• Locate
  "C:\Windows\System32\drivers\etc" for Windows and "/etc/" directory
  for Mac, then locate “hosts” file andOpen it in Notepad (for Windows)
  or TextEdit (for Mac OS X) and add this at the bottom of the file.

74.208.105.171 gs.apple.com

At last it will be like that, then save it
Step 4:
Now, You will have to restore your iPhone or iPod Touch to the firmware
you have downloaded by Clicking on iPhone icon that appears in the left
sidebar of iTunes then hold left “alt” button (“Shift” button on
Windows) on the keyboard and then click on “Restore” then release the
alt button, Now Select the firmware you have downloade din Step 1 and
Let iTunes do some automated Steps. (Note: If you got some error, Just
ignore it)
Step 5: You will need to Exit Recovery Mode, So Download RecBoot (Windows / Mac) and Click on "Exit Recovery Mode" and your Device will restart automatically.
Congratulations
Cheer, you now have your Device on 3.1.3 or 3.1.2, Now you will be able
to Jailbreak your iPhone 3GS / 3G and iPod Touch 3G / 2G using Spirit
by following the guide posted here.