The modulator is at the front drivers side of the trans (after the bellhousing). The adjustment is probably under a anti-tamper adjustment cap. If you could get to it there is a point which when squeezed would release the cap. In the car one will have to pop the cap with a screwdriver (possibly destroying it - have a spare).
Beneath the cap is a metal adjusting tee. Turn the tee 3/4 turn clockwise for stronger shifts.
A simpler adjustment is the control pressure cable. The cable housing has one adjustment. To make the shift earlier the cable housing will need to be adjusted shorter (the cable in effect made longer). This adjustment is a plastic, 13mm wrench size, nut at the cable attachment. By making the shift earlier it usually causes the shift to be made harder (as one doesn't tend to let up on the throttle).
Bosch Master, ASE Master, L1
33 years MB technician