How can i switch back to the old invoice template?

Welcome to the Community, @sg49.

The new user interface has been completely developed and has been introduced since June, offering new features and improved accessibility. Starting from October 16, 2023, Simple Start users will be automatically transitioned to the new invoice experience and will not have the choice to opt-out. It means they will be unable to switch back to the previous invoice layout. However, users who have upgraded or are currently using QuickBooks Essentials, Plus, or Advanced versions can choose to revert to the old invoice layout if they prefer. They can do this by selecting "Old layout" from the invoice screen. Rest assured, they will not lose any of their existing invoices. You can find the Old layout button at the upper right corner of the invoice page. See screenshot below.

Here are some resources that you can read about managing invoices in QuickBooks Online:

• See what’s new with quotes and invoices in QuickBooks Online 
• Create invoices in QuickBooks Online

For future reference, you can also check this article to help you record invoice payments in QuickBooks Online: Record invoice payments in QuickBooks Online.

Don't hesitate to comment below if you have questions about managing invoices in QuickBooks Online. I'm always here to help. Have a great day.