You can upload a world on the "worlds" page: https://exaroton.com/worlds/
Click the "Upload" button on the world you want to replace and select a ZIP archive or folder to upload it to your server. This is only possible while your server is offline.
World uploads are limited to 1 GB for technical reasons. If your world is bigger than that, you could try to decrease your world's size by removing unused chunks using a program like MCASelector.