This works best with uncluttered skies.

1. Open the image with the sky you want to remove. You now need to select the sky. This can be done with the Magic Wand.
The default tolerance on the magic wand is 32. You may need to change this – maybe to 20. Make sure Contiguous and Anti-alias are ticked.
Left click in the sky area. If some of the sky was not selected then hold down shift and left click in the unselected parts of the sky. If too much has been selected then hold down alt and left click to remove parts of the selection. (Alternatively you could use the Quick Selection Tool.)

2. Zoom in and scroll along the selected area to make sure all the sky has been selected.
You may want to expand the selection slightly. Go to Select>Modify>Expand. Choose 1 px and then OK.

3. Open the Layers palette. You should find just the Background Layer. This is locked and cannot be messed with. To edit it, you
need to hold down alt and double click on the Background Layer (or drag the padlock icon into the waste bin). This is now Layer 0 and you can edit it.

4. If you are not at full screen, double click on the Hand Tool or hit Ctrl 0 (zero).
Hit Delete or Backspace to cut out the sky. A chequerboard now appears where the sky was. This means it is transparent.
Get rid of the marching ants by hitting Ctrl D.

5. Open the image with the sky you want to use. Select all of it (short cut is Ctrl A), copy it using Edit>Copy or Ctrl C, and paste into the space using Edit >Paste or Ctrl V. There should now be 2 layers – Layer 1 and Layer 0.

6. You can use the Move Tool if you wish to put the sky roughly in position but it is still in front of main pic and it needs to be behind it.
Drag Layer 1 so that it is below Layer 0. The sky should now shine through.

7. You can now use Free Transform to adjust the sky. You may want to create extra space by hitting Ctrl - (dash) so that you can see the bounding box. Left click on Layer 1 (Sky Layer) to make it active. Hit Ctrl T (Free Transform).
You should now be able use the handles to adjust the sky to its optimum position, eg stretch or squash it. The mouse can be used inside the bounding box to move the sky around. When you are satisfied you can apply the transformation by hitting Enter or double clicking inside the image or left clicking the Tick.

8. Hopefully there will not be an obvious join between the sky and the main image. However, you might wish to check this. Zoom in to about 300% and scroll along the join. If you find jagged edges, these can be softened using the Blur Tool (looks like a teardrop).
Use a soft brush about 10 px, Mode: Normal, Strength about 50%.
Make sure Layer 0 (Main Image) is the active one and carefully brush along the join. Use the space bar and mouse to scroll along the join.
Hit Ctrl 0 or double click the hand tool to go back to full screen.

9. To improve the join still further you may wish to Defringe. Go to Layer >Matting>Defringe. Choose Width 2 px and click OK.

If you don’t want to make any other Photoshop improvements you can now Save your new version. You might want to crunch down the layers using Layer>Flatten Image first, but please be aware that this cannot be undone at a later date.