With the 'sky' image showing go to Edit> Paste>Paste as a New Layer. The 'foreground' will appear to be on top of the 'sky' image.

Now choose the Eraser Tool and begin to move it over the bright sky. To work effectively with each of the tools, be sure that you can see the Tools Options by going to View>Palettes>(and check)Tool Options (F4). You can see them here, for the Erasure Tool, just above the photo. You'll need to play around with the various settings.

Read your manual (or go to HELP) to learn more about various tool option settings. Be sure to see "Choosing brush options" which discusses the Shape, Size, Hardness, Step, Density, Thickness etc. including Blend Mode. Another very useful topic is "Using Tools".

The darker sky from the 'sky' image will begin to show through. Here I have almost finished erasing the bright sky but there is still work to be done.I need to choose a smaller brush to get closer into the too-light areas. And ....

... you may need to zoom your image larger to do a good job in this area. Go to Page 3