I happen to have a BDP-51.
You can change output resolutions by using the remote control and pressing the buttons labeled "output resolution".
Just an FYI, although I upgraded to an Oppo 95 (moved the BDP-51 to a bedroom), I've always thought that the BDP-51 has a more pleasing picture for BluRay. Only drawback is that it's a little slow booting and it doesn't do 3d which I can live with as I only have a couple of 3d movies. With that said, the Oppo 95 has very good DACs for 2 channel audio.
Samsung 64F8500, Panasonic 65VT50, Oppo 95, Tivo Roamio for OTA, Dish VIP722, Marantz AV8801 preamp, Rotel Amps, Atlantic Tech 8200 speakers, Seaton Submersive HP, Calman 5, Chromapure, VideoForge HDMI II, i1Display3pro, i1pro2, eecolor colorbox.