Visit your dentist. for the professional cleaning and polishing. If it is a removable denture, your dentist can clean it in ultrasonic solution first and then polish the surface with a low speed motor. Do not try to do it yourself, you can ruin the false teeth. Take care. .
Depends on condition. If the denture teeth are old and discolored, the best way to whiten the teeth is to place new ones. Sometimes, it is just superficial stain, which a dentist can place in an ultrasonic cleaner and polish if possible.
Fixed or removable ? If the teeth are made of porcelain, the can be polished to remove surface stains but will not change their color. Plastic teeth may have absorbed the stain over time and will look yellow, no matter what is done. Speak to your dentist regarding what might work best for you.