I have seen a lot of teeth whitening tutorials, and this is what I believe is generally the fastest and easiest method.
1. Open the photo you wish to edit.
2. Select the paintbrush tool.
3. Paintbrush Mode- choose "Saturation"
4. Paintbrush Opacity- 70% (Modify this level as necessary)
5. Paintbrush Brush- choose a brush with a soft edge (fuzzy circle), and choose a size that is similar in the size to the smallest yellow tooth.
6. Make sure foreground color is set to black or white. (important)