I assume your veneers are made of porcelain, in that case then you can not make them lighter without replacing them for new ones.
Maybe the veneer is too thin and the teeth below (your teeth) is yellow, so you can see like "behind the curtain" , in that case, there are 2 options a) replace the veneers for a lighter shade or get crowns which require a bit more of prep on your teeth.
There are two type of Veneers...composite and procelain. If you had your veneers made in one appointment in the dental office, they are probably of white material or resin. This is called a composite veneer. Your dentist can polish the composite veneer and remove most of the stains. If your veneers were made of procelain and, most are porcelain, there is no way to take the yellow color out of the porcelain. You will have to have them removed and replaced with new ones.
How to change color of veneers
The only way to make veneers whiter is to change them to a lighter shade. Usually the question of color is decided before the case goes to the lab. In your situation you should consult with experienced aesthetic dentist to achieve desirable effect.
The only option is to redo the veneers. Some veneers can darken is certain types of adhesives were used to put them on. They unfortunatly will need to be removed and remade.
Porcelain Veneers do not whiten.
If your veneers are made of porcelain it won't be possible to change the color without replacing them. However, if your veneers are made of composite or resin you may be able to lighten and remove superficial stains with different whitening products on the market and or having them polished professionally.
Veneers are yellow, can I whiten them
Unfortunately there is nothing that can be done to change the color of your veneers. The only option would be to replace them
Veneers will not change color
There is nothing that can be done to improve the color of the veneers, short of cutting them off and replacing them.