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.
You cannot. To change the color of a veneer it needs to be replaced.
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.
Impossible. You really need a new veneer.