Open up the invoice where the customer purchased the membership and click the 3 dots to the right of the item and select "reverse" so that the negative amount appears in the shopping cart.

Then go to the customer profile page that you need to add the family membership to. In most cases the parent should have the family membership invoiced to them so that any children under their account get added to the family membership.

Click the shopping cart/Buy icon under Current Memberships on the upper right hand side of the customer page and choose your family membership.

In your shopping cart, you will now have a negative and a positive item with an amount owed for the difference between the 2 memberships. This is what will need to be collected from your customer.

Press Checkout button and choose a method of payment for collecting the remaining balance from customer.

Hit Process Checkout.