Open the photo in Photoshop
Duplicate the background layer
Zoom in on the teeth
Select the teeth with the Lasso Tool, Quick Selection Tool, or Pen Tool
Feather the selection slightly
Add a Hue/Saturation adjustment layer
Reduce the Saturation
Increase the Lightness slightly
Adjust the selection mask if needed
Use a soft brush on the mask to clean edges
Lower the layer opacity if the whitening looks unnatural
Optionally use Curves or Levels for finer brightness control
Save the edited image