You will need to use an FTP client to upload the world/save .vsdbs file to the Saves folder on the server
https://hosthavoc.com/billing/knowledgebase/522/How-to-use-an-FTP-client-with-your-Vintage-Story-service.html
Once the file has been uploaded, rename your save file to be default.vsdbs
If there is already an existing default.vsdbs, you will need to delete the existing file.