I need to change a "bill payment" to a "pay down credit card" how do I do that since my payment already was reconciled?

I got you covered, Laura.

You'll want to undo your reconciliation to change your bill payment. This way, you can accurately reconcile your account again. I'll show you how.

1. Go to the Accounting menu in your QuickBooks Online (QBO) account.
2. Select Chart of Accounts.
3. Find the account you want to undo then select the View register link,
4. Select the transaction you want to unreconciled to expand the view.
5. Review the check column. If the transaction is reconciled, you’ll see an “R.”
6. Select the checkbox until the box is blank. This removes the transaction from the reconciliation.
7. Click Save and close the account register

You can also see this link for more details: Undo or remove transactions from reconciliations in QuickBooks Online. 

I'd also encourage you to work with your accountant to ensure the safety of your accounts after making these changes. You can also invite them to your account and let them undo the reconciliation for you.

Once done, you can now redo the reconciliation process whenever you're ready.

If you have other concerns or questions about reconciliation, let me know in your reply. I’ll be here more than happy to assist you. Take care and keep safe!