I'm here to lend a helping hand with reversing a credit in QuickBooks Online.
I can provide you with the easy steps on how to reverse a credit. As an initial step, let's turn off the option to Automatically apply credits before reversing it so it will not apply to other open invoices.
Click the Gear icon at the upper right corner.
Under Your Company, click Account and Settings.
Go to Advanced tab in the left panel.
Select Automation and remove the check mark for Automatically apply credits.
Once done, let's locate the credit and delete the zero payment to reverse the credit.
Double click the credit to open the transaction.
Click the payments made at the upper right corner and select the linked transaction.
Make sure that the amount received is zero.
Click More, and select Delete.
You'll now have the unapplied credit.
I've attached screenshots to serve as your reference.
Please let me know how it goes or if you ran into a different situation by leaving a reply below.