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 click on File Manager
2. Click on the Upload button
3. Select the .zip file from your computer and then click on Open
4. Unzip the folder inside the File Manager
Once the world is uploaded, you can assign it as your main world by using the World tab and updating your Level Name.
Just be aware that there is currently an upload size limit of 500MB.
You can also watch this tutorial on how to upload a world: