Removing the yellow from teeth is very easy to overdo. Take it easy on this one.
- Create a Hue/Saturation layer via Layer > New Adjustment Layer > Hue/Saturation Layer. On the top of layer palette, change the blending mode from Normal to Color.
- Select the white box next to the icon of the new Hue/Saturation layer and fill that with black (Edit > Fill...).
- Select the brush tool and paint white over the teeth you’d like to change. Nothing will change as we’ve not adjusted the Hue/Saturation yet, but paint it back in anyway.
- Back on the Adjustment Layer panel, change the colors affected from “Master” to “Yellow”. Reduce the Saturation level to about 15% and raise the lightness to remove black.