Changing a parent or child is not possible directly, you will have to delete an existing relationship and create a new one.

To delete relationships, you can click the Delete icon next to the parent account's name in the Customer page. You need to open the child customer in order to do this. 

You cannot delete a relationship if the child account has:

• Open Quotes
• Unpaid Invoices
• Unbilled Charges
• Net D Invoices
• Invoices in dunning