I'm here to help ensure you'll be able to reverse an invoice payment in QuickBooks today. First, I would like to know what QuickBooks version are you using. This way, I can gather enough details and give you the correct instructions.
You can locate the Sales tab as instructed by HoneyLynn_G above once you'll open QuickBooks Online (QBO) via a web browser.
If you're using an android app, you can follow these steps:
Log in to your QBO account, then click on the three horizontal lines (hamburger icon) from the homepage.
Tap on Customers.
Select the customer's name, then locate the payment you want to remove.
Open the payment and click on the three horizontal dots at the upper-right side.
Hit Delete, then select Delete again to confirm the action.
On the other hand, if you're using QuickBooks Desktop, here's how to delete the payment transaction:
Locate the payment from your customer's profile, then click it to open.
Select Delete, then OK.
I've also added these articles to learn more about deleting or voiding transactions in QuickBooks: