When the sky you shoot is not you want, like a gloomy sky, but the other object is so beautiful that you don’t wanna delete, you may be hope to replace a sky. Musemage can help you quickly realize this idea.
Look, this is the original picture. The sky is gloomy, and I want to replace another one.
And the following is a replaced sky effect.
Looks like a different style. Now, let’s get started to learn how to make this.
First, open the original picture, and open the other as a new layer.
Then, use Free Transform Tool to make the sky over the original sky. Change layer Opacity to 50%, so that you can see your processing carefully.
Next, add a Mask to the second picture, and choose Brush Tool to erase the unwanted part. 
Change the opacity of the brush tool to 20% when erase the sky’s edge.
After erase, adjust the layer opacity to 100%.
Now, the effect comes out.