Open the back cover, and look on the flyback transformer. There may be two or three pots. One will most likely be the beam focus, and the other(s) will be the focus adjustment.
In some sets, the focus control is on the CRT circuit board.
Before touching anything, read carefully what the adjustment does. Use only a plastic or Teflon tip (non conducting) tool to adjust these pots. Using the fingers can be risky, if you are not used to this type of work. If you use a conductive type tool to do the adjustment, and you slip, there is a very good possibility to do circuit damage, as well as possibly get hurt.
If the monitor or TV sets requires the focus to be touched up, this is a sign that either the CRT is going weak, or that there is something failing in the high voltage section.