Open your image in Photoshop and zoom in on the teeth that you want to whiten.
Create a new layer by clicking on the "New Layer" button in the Layers panel.
Select the "Brush" tool from the toolbar and set the opacity to around 50%.
Choose a soft brush and adjust the size as needed to fit the tooth you want to whiten.
Select the new layer you just created and start painting over the teeth with the brush tool, being careful not to paint over other areas of the image.
Once you have covered the teeth with the brush tool, go to the "Image" menu and select "Adjustments", then choose "Levels".
Adjust the levels slider until the teeth are bright enough, but not too bright that they look unrealistic.
If you find that the teeth still look yellowish, you can adjust the "Hue/Saturation" by going to "Image" "Adjustments" "Hue/Saturation" and dragging the saturation slider to the left.
Zoom out and check the overall image to ensure that the teeth now look natural and not overdone.
Remember to save your edited image as a new file, so that you always have the original copy if needed.