I'd be glad to help you out by sharing possible scenarios I could think of.
Are you referring to a transaction that was entered incorrectly? If so, I can show you the steps on how to delete or void the transaction.
You can also void and delete a transaction from the All sales.
Please feel free to read this article for your reference: How to Void or Delete an Invoice or Other Transactions.
But if you're trying to process a refund, you can follow these steps to record it in QuickBooks Online.
You can also read this article for your reference: Create a Refund Receipt.
Feel free to leave a comment below if you're referring to something else or if you have any other questions. Have a blissful day!