If an invoice was created and I changed it by accident, how can I undo the last action?

You can edit your invoice and make modifications, @hubprofi. I'll go over some details to explain it further.

If you've already saved the changes to your invoice, there's no direct way to undo the last action. However, you can manually edit the invoice to revert it back to the original state. See the screenshot below for your visual guide.

To help you keep track of any modifications, you can use the Audit Log in QuickBooks Online.

Moreover, you may visit this article to learn how to record your sales transactions: Record invoice payments in QuickBooks Online

If you have more questions or concerns about managing your invoices, @hubprofi, simply leave a reply below and I'll provide the necessary help.