Whether you want to jailbreak, un-jailbreak, update your device from expired beta, or need a solution when nothing else works, Apple has a solution for you - put your iPhone, iPod touch, or iPad into DFU (Device Firmware Update) mode.
DFU is an abbreviation for Device Firmware Update. This is a special mode allowing you to connect your iPhone to iTunes on Mac or PC without calling the iBoot process. Since this method bypasses iBoot, DFU mode can be used to update iOS, roll back and even reboot devices with a jailbreak attached.
While in DFU mode, your iPad screen will remain completely black. If you see Apple logo, you did not enter DFU mode, so you will need to start over again. If you see Apple logo or iTunes logo, you probably entered into Recovery Mode instead, which can sometimes work for restoring. The key thing to remember about DFU mode is that screen remains black, but iTunes alerts you that your device connected to PC is ready to restore.