1. Click on a member of your Tree.
2. In the Sidebar, click the "Change or Delete" button.
3. Next, click the "Change parents" button.
4. Next, click the "Choose new parents from tree" button.
5. Select Mother from the list, then Father from the list. You can also select one Parent, leaving the second one the same.
6. Click "Set new parents" to save, or "Cancel" to exit without change.
Good luck