How Do You Clean Stained Composite Veneers?

Composites can be polished, but this is not likely to be a one time permanent solution. If they are stained after only one year, they will usually continue to stain and need to be re-polished periodically.

Composite veneers are porous and pick up stain easily. Porcelain is not porous and the stain does not adhere to them. I would go to your dentist and haev him try to "buff" out the top layer of stain with special disks, But you will just keep returning to do this.
Your best option is to do porcelain which are much more lifelike, won't stain and last longer.