I want to edit existing invoice.

I'll walk you through the steps, @adlanashrafoffic.

All you have to do is go to the sales menu. Here's how.

1. Go to Sales.
2. Choose the Invoices tab.
3. Change the Status drop-down to All, and then select the Date drop-down.
4. Locate the invoice you want to edit. Click View/Edit under the Action column. See the image below.

Learn more about managing customer-related transactions at this link: Manage Customers and income for QuickBooks Online International.

Please don't hesitate to reply to me anytime if you still have questions or concerns with invoices. I'll be around for you. Take care and have a wonderful day ahead.