Now that you've removed the parents for the existing profile, you need to add the profile as a parent for someone else.
First, make a note of the profile number for the person you're working with (e.g. Smith-12345).
Go to the profile of the child of Smith. Click on the "Edit" tab. On the right side, under "Edit Family" click on the spot for "Add father."
A new page will open and all you need to do is type in the complete WikiTree ID for the person you are adding as a father (Smith-12345). Scroll to the bottom of the page and click the big green "Add father of ____."