There are two scenarios I can think of when you say "sales invoice format". If you mean QuickBooks Online (QBO) auto-fill the invoice info when choosing a customer, you'll need to do a few tweaks on the Account and Settings page to disable it.
Click the Gear icon, and then choose Account and Settings.
Select the Advanced tab.
Click the pencil icon on the Automation section.
Turn off the Pre-fill forms with previously entered content radio button.