There are two actions that you can perform on an existing server, renaming it or deleting it. These actions can be found by hovering over the server on the Servers page.
To rename a server, simply hover over the server and click 'edit name'. This will display a small modal that allows you to change the name.
To delete a server, hover over it and press 'delete server'. To protect the servers from accidental deletion, you must confirm the name of the server that you are deleting.
The agent will then uninstall itself from the server after it has finished any remaining jobs that may have already been started. For more information see uninstalling the agent.