How to upload world to Multiplayer
There are two ways to upload your world to Multiplayer.
Upload it from Multiplayer Mode
1.1. Open Multiplayer Mode and click on "Upload" button.
1.2. Enter the world's name, enter the password (if you want to make the world with password).
1.3. Choose your world from the selection, that you want to be uploaded to Multiplayer.
1.4. Tick a read-only box (if you want to make the read-only world)!
1.5. Click on Upload button that located on the right bottom.
1.6. Search recently uploaded world in Multiplayer by clicking on the "Search" button.
Upload it from Singleplayer Mode
2.1. Click on "Upload Online" button.
2.2. Follow all steps from 1.2 to 1.6 above.
To protect your own worlds in Multiplayer, please read this tutorial thread:How to protect worlds in Multiplayer