If it's not a jailbreak iPad there should be no problem. If there is, and there's an Apple store nearby, they'll always help you out. Plus - if you sync the iPad on iTunes before you upgrade, you should be able to restore it if there's a problem.
My iPad was iOS 4.2 when I purchased it, so I shouldn't be commenting on this, as I've never had to upgrade myself but, reading the Forum posts, most people seem to accomplish this with little trouble.
Tim
Scotland