Take control of your scene by mixing and matching people, buildings and backgrounds
Use Quick Selection to select the sky. If there is good contrast between the buildings and the sky, it should be painless. Again, use Opt/Alt to remove areas.
Zoom in to examine your selection. Hold the Spacebar then click and drag to pan the image while zoomed. Create blocky selections with the Polygonal Lasso.
Once the sky is fully selected, hit Delete/Backspace to get rid of it. If you just want to hide it, invert the selection with Cmd/Ctrl+Shift+I, then add a mask.
Now you can slide in the new sky image beneath the city. Use colour and adjustments like Levels, Vibrance and Colour Fill to make everything blend better.