Upload a mission
Go to "Sandbox" menu inside the game, edit a mission/create a mission and press "share" when it is ready.
Download a mission
Subscribe to a mission in the Workshop page of the game, then restart the game.
Delete a mission
To delete a personal mission just delete it on "Sandbox" menu in game.
To delete a downloaded/uploaded mission, unsubscribe from it on the steam workshop page of the game, then delete it from the "Sandbox" menu in game.