I have had dentures for several years. The only problem I am having with them is that they are starting to look yellow. How can I whiten them up.

Welcome to the board.

It depends on what is causing the yellowing as to whether you can get them white again. What are you using to clean them? Do you smoke? Do you scrub them with a brush or just soak them?
Try brushing them with dishwashing liquid- most are made to cut through organic matter and that may be what is staining them. Of course take them out of your mouth before using the soap!
If they were not particularly "good" denture teeth they may be porous and allowing material to get into the surface of the teeth. Talk to the dentist too.

Try an overnight soak in hydrogen poroxide..it might whiten them up.

Thanks, I guess I should have given more info. I do brush and soak my teeth everyday. And I am a smoker. I also have tried soaking in peroxide