There are a couple different ways to change your server so it uses a new world. You can use either of the following methods.
Rename existing world
Want to keep your world in your files to use it later? Stop your server, go to the Files tab, and rename your "world" folder to something else, like "world_old". Then, once you start your server again, you'll spawn in a new world!
Alternatively, if you don't want to keep your world, you can hit the Delete button. Please note, however, that it will not be recoverable if you do not have a backup saved.
Change level name in server properties
Don't have a folder named "world"? Go to the server properties tab in your server options, then search for "Level Name". Change the value to something else, then make sure to "Save and restart". Once your server starts back up, you should have a new world!
