Thanks for reaching out to us!
There are a few ways to achieve this. One easy way, is to use the Effects tab > Sketch tool to paint blue over the sky. Change the “Blend Mode” dropdown to “Multiply” and Fade the effect so that it’s very subtle. You’ll need to find the right color for the sky, probably something like #0099FF will look good once faded and blended.
Let us know if you have any further questions!