You can click the Save button after making changes to the invoice. However, if you already sent the existing invoice to your customer, you'll want to resend it. This way, your customer will be updated on the changes made in the invoice. Let me show you how.
In your QuickBooks Online (QBO):
Go to the Sales menu.
Select the Invoices tab.
Locate and open the invoice you want to edit.
Click the (delete)Trash icon from the invoice to remove a line item.