datasheet:
Calibration
The multimeters of the METRAHit S series can be calibrated automatically via the RS-232 data interface with the aid of the multifunction calibrator METRAtop 90C and the METRAwin 91 software.
Gossen doesn't publish calibration procedures, and you need RS232 cable. That's bad news. Good news is that in Germany you should be able to find a company that is able to professionally calibrate it. Trick is to get them to do it cheaply.
If you don't mind me asking, why do you want to calibrate it? They are good quality instruments, they don't drift that easy. If you verified it gives incorrect readings, it is more probable it was damaged than drifted if error is large. So you would first have to repair it before checking accuracy any further ...