In build mode at the bottom of the menu is the world tab. This is an important page where you can access publish, player settings and capacity tabs.
Within the publish tab you can change the world’s name, set a thumbnail, write a description and select a tag. World tags allow your world to be categorized and more easily discovered. You will also want to select a comfort rating. This lets people know ahead of time what kind experience they can expect. Scrolling down, you can change whether the world is listed as a beta world, and make it visible to the public or unlisted.
Please see the publishing tutorial for tips on publishing your world.
WELCOME TO BUILD MODE SETTINGS precedes this tutorial
Seen above is the world tab found in the build menu.