Service manual is located here :https://www.elektrotanya.com/sony_pvm-20m2_20m4_a-e-u_sm.pdf/download.html
Press menu, and then ENTER and DEGAUSS together while the menu is open to access service.
Be Aware that the H Frequency (position), V Size depend on if you're using a 50 or 60 Hz source, so they are under two different headings.
NOR DEF has most of the picture adjusting functions, with NOR 50 DEF or NOR 60 DEF with the ones mentioned above.
The changes you make are only live till you power cycle the monitor. To make them permanent you have to write RAM to ROM.
To do this :
While in the service menu, press Degauss once, a star and WRITE will appear. Press Degauss again and it the RAM will be written to ROM.
Don't do a factory reset, it's a complete ball-ache to get up and running again.
Pages 16/17 of the above document have what you're looking for.