This video illustrates the method of Downgrading the iPhone 3Gs firmware from version 3.1.3 to version 3.1.2. The process is composed of the following steps -Step 1Download the iPhone 3Gs firmware version 3.1.2 from the internet. You can find this on apple's official site or you can get this from a customer service center.Step 2Upload the firmware files in the root of your iPhone's internal phone memory. Create a folder named "firmware v312" and add the firmware files in it.Step 3Access this folder from your iPhone and start the firmware installer. Click on the install button and wait for the installation process to complete. When you are done , reboot your iPhone. To check the firmware version , go to phone information tool. That's it.