It never hurts to check on the steps for how to complete tasks in your QuickBooks Online account, and I'd be happy to guide you on creating a customer refund in the program. It's a fairly simple process that's completed in just a few steps.
For the most basic kind of refund—returning a customer's money for a returned product, for example—all it takes is going to the +New icon in your books and choosing Refund receipt. This doesn't link directly to a sales receipt or payment transaction, but will show that the funds were returned to the customer for a given amount. This article gives you the step-by-step: Record a customer refund in QuickBooks Online. This article is also useful if you have a customer overpayment or a credit that you'd like to refund them.