Thanks for posting your concern, Aqua. I'd be glad to walk you through to switch back to estimate.
To return it, click the X icon at the top right corner on the invoice page, then go to the +New icon and click Estimate. Refer to the sample screenshot below:
Additionally, check this article to help you personalize the appearance and layout of your sales forms: Customize invoices, estimates, and sales receipts.
I'll be around if you have other questions about any transaction concerns. Take care.