You can't. In order to make the crown whiter, the dentist would have to take it off and send it back to the lab, where the porcelain would have to be stripped off and new, lighter porcelain applied. The problem with that is, in order to get a crown off once it's been cemented, the dentist usually has to cut through it, destroying it. If you need a whiter crown, you have to get a new one.