When you have finished making changes to your world, then you're almost ready to publish. As a final step, make sure you play through your world in "test mode".
When you have finshed testing and everything works as expected, you're ready to publish! Tap the PUBLISH button on the bottom-right corner of the screen in the World Editor.
Editing your world
Once your world is publish, you can edit it at anytime by going to your "Home" screen and tapping on the world.
When you tap on the world, you will open the world screen, which contains the option to "Load Editor", as well as other options related to the world. "Load Editor" will open the build view and allow you to continue editing your world where you last left off.