New world

[cyan plover]

how do I start a new world without creating a new instance or is that impossible?

[golden monolith]

if you don't care about keeping the current world: Stop the server, head to the files manager, delete the world folder, start the server again.
if you do want to keep this world: Stop the server, head to the home page, change the level name to something else, hit save, start the server.