The Default World, also known as the Level Name is the world players will join in when connecting to your server. The default world is "world". You can change this by uploading your own world, and then following the steps below.
First, open your Server Dashboard. Navigate to
Edit Server / Settings / Level Name
The Level Name will already be set to 'world'. Change this to the name of your desired default world. Make sure you've already uploaded your own world before changing this setting.