Hi there, @hannah-martinez-.
If the payment hasn't been settled yet, you can create an offsetting refund receipt to reverse the payment. To create an offsetting refund receipt:
You can also refer to these articles for more information and detailed steps:
Also, make sure that you're able to process the refund before the settlement cutoff time (3 PM PST on the same business day). If the settlement cutoff time has passed, you'll need to get in touch with our Merchant support team. They have the tool to pull up your account in a secure environment and help you process the cancellation.
To know some tips about recording your customer's payments in QuickBooks Online, you can always visit our Payments page for reference.
If you need more assistance, please don't hesitate to leave a comment below. We're always here to help.