How can I exit / get out of iPhone DFU mode?
As long as nothing is actually wrong with your iPhone, you can exit Device Firmware Update (DFU) mode without restoring the device in iTunes. To get out of DFU mode follow the same steps as a forced restart.
1. Hold down the Sleep button and Home button at the same time.
2. After a few seconds the Apple logo appears and the iPhone will boot.
3. Your iPhone is now out of DFU mode.
If the procedure doesn't work the first time, try steps 1-2 again. If something is wrong with your iPhone you may need to restore using iTunes.