How can I whiten my crown?

Unfortunately crowns do not whiten at all.  If you are desiring the crown to match your teeth you will need to have the crown replaced with a new shade crown that will more accurately match the existing teeth.  Please keep in mind that if you want to whiten your natural teeth, do so prior to the new crown so that way the replacement crown will match where you want to keep your teeth shade/color more long term.

You could drink lots of coffee or smoke cigars to make the other teeth yellower, or you can change the crown to one that matches.  Something to consider -- if it is one of the two front teeth, consider having the other tooth veneered with the same porcelain so they will match over time, as you probably will drink coffee and smoke cigars (at weddings and golf tournaments) and your natural teeth will yellow and before you know it, the crowned tooth looks too light and out of place.

Unless the crown has some staining the shade of the crown cannot be changed. If the crown is stained your dentist can use a pumice paste to remove the stain. If the crown is not stained, unfortunately the only way a crown can be whitened is by replacing it.  When a crown is fabricated at the lab, the porcelain shade is mixed and the material is initially liquid-like. It needs high temperatures to cure and harden. Often these temperatures can reach up to 2,372 °F (1,300 °C), and sometimes dental crowns need to be heated for up to eight hours to ensure they have been completely set and cured.  For these reasons, when the crown comes back from the lab and is cemented to your tooth, the shade cannot be changed.