Before you can change an invoice, you need to cancel it.
This is necessary in order to ensure that all sessions and ad hoc purchases are included.
Go to either Batch Invoice (in the Revenue & Debt app) or on the child's profile and click the invoice you need to cancel. The invoice opens in a new window and gives you the option of cancelling the invoice by clicking the three dots in the top left of the invoice.
You can now make all the changes to a child's plan and ad hoc purchases you need, reissuing a new invoice when these changes are complete.