Put simply you cannot, if all you have on the projector is a 15 pin D-sub in blue then the maximum resolution you can get out of it is the quoted native resolution.
Native is the highest, screen size input and pixle resolution, plus the screen format the projector is capable of displaying. In your case 4:3 dimension.
HD requires a 16:9 or 16:10 dimension.
Anything above it, from a source device such as a Pc output will yeild a message on no signal, a blank or blue screen because it is beyond what the projector can produce in image resolution.