Firstly look at the external controls for a height adjustment. If no luck and you feel confident that you won't kill yourself, on the "chassis" there will be a height control. It may be marked vert(ical) amplitude or some other way of saying the same thing. Try simply swinging this small variable resistor through its range several times, then adjusting the height -- with the aid of a mirror if needed.
The high voltage on colour tubes is seriously nasty...don't get zapped. Use a knitting needle cut to a screwdriver shape if the pot is in a difficult place.
Often the screw either side of the video in wire will also have to be removed. Replace all screws in the holes from which they came. Good luck