Work with the Burn tool

From the toolbar, click and hold the icon for the Dodge, Burn, or Sponge retouching tool, whichever is active.

Select the Burn tool () from the available options.

Choose a brush tip and set the brush options in the options bar.

In the options bar, set options for Range, Exposure, Airbrush (), and Protect Tones.

Paint over the parts of the image that you want to darken.