4. Double-click the background layer. You will now find a New Layer dialog box. Press Enter.
5. Simply drag this new layer and drop it in Layers Palette. Now this layer will be at the top and the sky will be at the bottom.
6. Find the Blend If area now. Slowly and steadily drag the slider to the rightmost corner. You can now find the areas in the sky that are very light.
7. To ensure that both sky and other parts of the image are intact, just hold down the Alt key and then simply drag the slider in such a way it splits into two parts. What happens now is that values that are on the left indicates no blending is done, right indicates complete blending and values between these left and right of the slider indicates partial blending.
8. If after all these blending, you find the image to picture perfect click OK. (Ref: Fig 3– Image with replaced sky)
Image Source: ProjectWoman
You have now successfully replaced the sky using Photoshop. Enjoy your photograph.