Uploading your creations
First of all, you need to make your level (in the level editor) After the level has been made, go to the "FILE" tab on the top, and hit "Share Map"
As easy as that!
Downloading others creations
To download custom levels, go to Steam's Workshop. Find a level you would like to download, and hit the "subscribe" button. After this, launch your ZKW-Reborn, and Steam will automatically load the level for you! You should then get a notification from ZKW that it has downloaded the map successfully. If not, just try to relaunch your ZKW, or Steam!
Remember to run Steam as Adminstrator (and ZKW) if you're having any problems.
How to update existing map?
At this point, you can't. It is recommended to upload your map with a slightly different name (for example. "My Map Version 2") and delete your old map (if you wish to do so)