Good to see you here in the Community, supremeklean1.
Splitting or changing a closed invoiced is unavailable in QuickBooks Online. You might want to delete and recreate the invoice, and split them before closing.
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I'm leaving you the following write-ups below. It contains more information on how to record multiple and partial payments in QuickBooks Online as well as steps to use progress invoicing:
Set up and send progress invoices in QuickBooks Online.
Record invoice payments in QuickBooks Online.
I want to ensure you're all set, so please let me know if there's anything else I can help you with, supremeklean1. Cheering you and your business to continued prosperity.