How do you whiten a porcelain veneer on your tooth?

You Can't. A porcelain veneer is color stable, and unfortunately immune to conventional teeth whitening products. If there are coffee or tea stains, they can be polished, but only a new veneer, with a brighter porcelain base, will make it whiter.
Hard to do. Porcelain itself will not whiten. You can slightly whiten the neighboring teeth and that can give the illusion that your whole smile is a bit lighter. Have your dentist check to make sure your veneered tooth is not actually darkening because of a nerve/abscess problem.
Whiten First. Although the veneer will not whiten, the tooth behind it can. I would recommend whitening with a profession gel before you consider a new veneer to lighten all of your front teeth if you haven't.
Whiten a veneer. How long has the veneer been on? There are ceramics available today in a vast array of shades. Generally once a ceramic restoration is in place the only successful attempt to significantly lighten it would be replacement. What is the reason that the veneer appears to l\need lightening? This is also an important issue. Do you have other veneers? Need to see to tell what is best.
Depends on your goal. A tooth with a veneer can be whitened if the veneer is translucent. Some veneers and bonding techniques limit the ability to whiten due to material that limits color transmission. Replacement may be the solution. See a dentist with cosmetic experience.
2 options. The porcelain itself cannot be changed to a different shade unless remade at the laboratory. Sometimes they can pick up stain or plaque that can be polished off at your dentists office. Clearly i would recommend that first before remaking a veneer.
Best way. Best way is to schedule for a cleaning porcelain can not be bleached if you are not happy with the color after cleaning , you need to replace the veneer.
Not easy. The porcelain veneer itself cannot be whitened, however if the veneer is thin enough, and shade is influenced by stump shade of your natural tooth behind the veneer, it may be possible to bleach the stump shade of the tooth, and ultimately the overall tooth shade will be whiter.
Replace the veneer. Veneers are made from porcelain or ceramic. You cannot change their color. This is why matching their proper shade is so important when the veneers are made. If you have decided to have whiter teeth, then you need to replace all the ceramic in your mouth. Bleach your teeth to the desired shade; wait a few weeks, then decide with your dentist how to match your veneers to your teeth.