The Brush Tool lets you blend the image with a source photo in a specific area. To do this:

1. Adjust the Opacity on the left panel (set to 100 to completely replace with the source photo).

2. Click and drag on the image to blend with the source photo.

To apply different blending adjustments to another part of the image, click on New Selection.

Close this window when you've finished adjusting the selected area. Any further blending adjustments will then be applied to the whole image.