Water changes

I have an eshopps overflow box that hangs on the back my problem comes in when I have do a water change. First i stop the siphon and stop my return pump. Now would I take water out of the display or sump ( no refugium in the sump)?. After that I would replace the water than how would I start the siphon and return pump so I can get my water level back to normal



PLEASE HELP