Are you referring to the focus? There is a knob on the flyback. Depending on how your chassis is mounted (some of the countertops have it upside down), you may have a hard time reaching it.
If you don't know what a flyback is, look for the suction cup attached to the tube (don't touch it), follow that wire back to the monitor chassis. It goes into the flyback. Look for a focus control on it...