OK, I still recommend use iTunes.
you can try this:
1: Connect the iPhone to your computer via the USB cable.
2: Put your iPhone in DFU mode
To do this, press and hold Home & Power button for 10 seconds.
After that, release the power button but keep pressing the home button.
iTunes will popup saying it detects a device in the DFU mode
3: If all is successful, you can "Restore" your iPhone now.
Tips: What I must say is that this way is not suitable for those who haven't backup their data. And if you get errors when you try to restore, you may need other tools.
Hope it is useful.