First step is to realize that rather than having a free app, you have an app linked to a free node.

The documentation that you are referring to is likely this: https://docs.mendix.com/developerportal/deploy/licensing-apps#unlink

In three steps generally:

• Get your licensed node.
• Unlink you app from the free node, link it to the licensed node, as described in the docs
• Restore a backup from your app.