Start with a selection.

Use the Rectangular Marquee tool to make a rough selection around the sky. Then press Control+C (Windows) or Command+C (macOS) to copy it. Then create a new file (File > New) and use Control+V or Command+V to paste the layer.

Take the stars for a spin.

An action includes a series of changes that can be made to an image with the click of a button. With the pasted sky layer selected, open the Actions panel (Window > Actions) and click the flyout menu. Choose the Star Trails Rotation action from the Stars Trails folder and click Play. Then save the file to your device as a JPEG.

Change the sky.

Return to the original image and choose Edit > Sky Replacement. Click the Sky thumbnail, and then click the + button to import the sky image file you just saved.

Fine-tune it.

Adjust the settings to finesse the results and click OK. Now that you have your new sky, you can use it in other compositions too.