You can download a free pdf copy of the service manual from
this URL.
According to the manual download, potentiometer RV501 on the "A" circuit board (the main board mounted on the bottom of the unit) is the vertical hold control. Unless there's a little hole to access it through the case bottom, you'd have to disassemble the display to adjust it. Since this is a CRT screen, a certain degree of caution is prudent when adjusting with the case removed, both since the CRT is breakable (and under vacuum), and since there are some quite high voltages present.
The manual doesn't seem to give much of a clue as to where on the circuit board this potentiometer is located, unfortunately.