To enter Recovery Mode:
Press simultaneously buttons "Home" and "Power" button.
The device will display the power off slider, then the screen goes blank. Do not release the buttons.
Wait until the device starts to boot and then release the "Power" button.
After 10-15 seconds, the screen displays an image of the cable and the iTunes icon.
Release the "Home" button
You entered the recovery mode. Connect your Iphone to ITunes and upgrade the firmware.