It's a complicated answer ... It involves assembly language and binary encoding...
First ... you hit escape to bring up the menu.
Next, click on the button that contains the text "Video Options"
Next, under "Resolution" you choose the new desired resolution.
Hmmm... wonder what happened to the hard part???
Oh wait ... that was self-explanatory ........... and in the manual in case you were really clueless ......... But heh, just post it here ... why not...