Do you have a sight glass on the bachwash line (bubble looking thing)? If so, backwash until the water in the sight glass is clear. If not, backwash for about 2 minutes, go back to filter mode, check your pressure to see if it has returned to the "clean" level, and repeat as necessary (though 2 minutes of backwashing will usually suffice).

Make sure that you shut down the pump each time before moving the handle on the multiport valve.