Edit just one part of an image, based on contents:

- open image
- Layer -> Duplicate layer
- in the layers window (use Windows at the top and check Layers, if you cannot see it)
select the duplicate layer
- Layer -> Layer mask -> Reveal all
( you will now have a blank layer mask. Essentially the picture will not have changed yet)
- Image -> Apply image
( now, in the layer window, that icon for the mask looks exactly like the picture, but black and white)
- edit the layer mask ( hold the alt key and click on the black and white mask )
- now, use the levels (image -> adjustment -> levels) to increase the image contrast,
and/or paint the areas that you wish to hide in black, and areas to reveal in white.
- click on the colour image of the duplicate layer
- make the adjustment, such as:
Filter -> Blur -> Gaussian blur

This change will only apply to the layer where there are white areas in the mask.
The beauty of this method is that it never touched the layer underneath.
The change can now be blended to soften the effect.

If you want to save this image as a JPG, don't forget to flatten the layers:
- Layers -> Flatten layers
Once flattened and saved, you cannot undo changes.
Make sure you save the file with a different name.