DFU mode

1. Connect your device to the computer.

2. Press and keep pressing the 2 buttons for 10 seconds.

3. Release the sleep button, but keep pressing the home button for 15 seconds.

After this, the device should be in DFU mode with the screen in black.
If the device is in DFU mode, your computer should have told you.

1. Press the 2 button until the device turns off.

2. Press the sleep button until you see this:

Now wait until your device restarts.