If you no longer need a server and want to delete it, you can do that on the servers page. Please be aware that this will delete your entire server and all data stored on it forever, and there is no way to undo this. If you want to keep your world and other data on your server, make sure to have backups. Backups are not deleted when you delete your server.
First click the edit icon in the top right:
Then click the trash button next to the server you want to delete:
If a server is owned by a different account, it will show an X instead. Clicking that will not delete the server and instead only remove your access to the server.