Uninstalling scenery can be done in two steps

First, launch Flight Simulator, and open the scenery library.

Highlight the area you want to delete, and click the delete button on the right. Flight Simulator will now reload the scenery, but the area you just removed will no longer appear. This removes the files from Flight Simulator, but they are still stored on your computer.

Next you must launch Windows Explorer, and find the folder where all of your scenery is stored

This may be a folder with the other Flight Simulator files, or may be a separate folder as shown above.

Highlight the folder you just removed from the Scenery Library, and press delete.