Remove the multi port control from the top of the sand filter
remove the sand inside the unit, this can be a next to impossible job to do by hand as they are not really built with this job in mind, however help is at hand with a little venturi pump that hooks onto your garden hose that will suck the sand out of the unit without having to scoop it out see your pool shop about getting hold of one of these.
When its empty replace the sand or whatever media you are using and screw the port back on.