To upload your existing Valheim world you will first need to login to the Management portal. On first signup you will receive an email with login details to your panel.
First, take a backup of your world to avoid any data loss of your existing world if required.
Next you will need to click on the "File Manager"
Navigate to Saved > Worlds and upload your existing world files. Take note of the name of your save as you will need it later.
Follow this tutorial https://quantumcore.com.au/clients/knowledgebase/30/Change-your-Valheim-server-nameorpassword.html and when setting the "Worldname" in the Commandline Manager, put the name of your world save.
Reach out to support if you get stuck. We are happy to assist.