How can I calibrate a reed Clamp Meter ST-332 ?
I don't know once the instrument is open, the right thing to adjust.
I own a clamp meter than can be calibrated from the panel, but this one have not that option.

If this is for industry and not for home use, then you should be sending it away to an accredited calibration service in order to get an official calibration certificate.

Send it off to a UKAS lab unless you have the calibration standards tracable to national standards. Try TER Calibration, Farnell, RS