You will get an email when a new one is available. At that time you can either download a zip file or use the itch.io app to update the game!
Right now there's no way to make custom levels, but next update will feature Endless Mode with a slew of new levels!
The level from that screenshot was an old prototype that is no longer in the game. You can see the emperor in the background looked a bit more like he was made up from some hastily arranged cubes. :)