How do you fix a CRT monitor showing a partial image?

If by "partial image", you mean that part of the screen is no displayed, use the knob / setting that looks like two squares on top of each other to move the image back into alignment.

If you mean that it displays an image even when that object is nothing there, you are suffering from the effects of burn-in. Burn-in may correct itself over time. To try to fix it faster, you will want a display of nothing but pure white on the screen, and leave it sitting for several hours, or even days.