This is something best left to a pro or you can end up with an awful mess. Have a look through Yellow Pages to see if you have any camera repairers in your area, who should be able to do the job in five minutes. The 20D has a removeable screen but don't be tempted to remove it. As it stands now the dust is probably on the outside (underside) of the screen. If you have difficulty getting the screen back in (and it's highly likely you will) you're likely to end up with dust, or worse, fingerprints, on the inside of the screen -effectively inside the prism housing. The inside surface, unlike the outside is a finely ground surface and will retain prints which can't be polished off.
Replacement screens come supplied with tweezers to assist fitting, but it's still a potentially troublesome job. I've made all the mistakes, so take my advice and don't do it.
