Here are the instructions on how to upload a world:
Make a zip containing all the files inside of the world folder. You can do this by selecting all the files and "archiving" or "zipping" all the files. Please be sure you don't name your world with any spaces.
1. Go to your server's panel and scroll down to the World Settings section.
2. Click on the Upload Zip button
3. Select the .zip file from your computer and then click on Open
Once the world is uploaded, you can assign it as your main world by using the Settings tab and updating your Level Name.