Do /worlds in game to see the list of your worlds
To upload a world: /ul world <name> <direct download link>
To download a wordl /dl world <world name>
Wait for it to compress the world, then it'll give you a link to download it as a zip file
To change the main world of your server:
Go the minehut server website
Go to Server Properties
Click Level Name
And the exact name of the newly uploaded world
*** Changing the level name will not affect your inventory nor the structures you've built in the world since the /dl command downloads everything.