Thanks for getting back to us, SY Chan.

Once you revise the Bill, you'll have to manually adjust as well the previous quantity on hand of the previous receipt.

Once done, you'll have to adjust the quantity on hand recorded on the system. This way, you'll have the correct tracking of the items on your QuickBooks Online account.

You can take a look at this article as a guide when adjusting the quantity: Adjust Inventory Quantity On Hand in QuickBooks Online.

Feel free to get back to this thread if there's anything that I can help with. Keep safe.