Go Layer>New Adjustment Layer>Vibrance and reduce the Vibrance to “-60”.
Next select the Vibrance layer in the layers panel and specifically choose the layer mask. In the Properties Panel (Window>Properties) select the “Invert” button to fill our mask with black to hide the desaturation that we just imposed on the Vibrance layer.
Next select the Brush tool (B) and I’ll grab a small, soft edged brush and look to the control bar across the top of the screen and set the Opacity to “50%”. Paint over the teeth you’d like to whiten for a realistic and beautiful smile!