Begin by downloading the calibration code from here.
Extract the zipped file onto the root folder of a blank USB stick. Note: You may need to fully format this USB stick.
With the printer turned OFF, insert the USB stick. Then switch the printer ON.
Tap the small crosshairs that appear on screen, until you hear three beeping sounds. Then turn the printer OFF once again.
Unplug the USB stick, and switch the printer back ON. You should now have a fully calibrated touch screen.