Changing the default name of the filed label is not possible. You can utilise one of our custom fields instead. Let me show you how.
In addition, here's an article you can read to learn more about customising the template of your sales transactions: Customise invoices, estimates, and sales receipts in QuickBooks Online.
Lastly, you can also read through the content of this helpful article for the steps in changing an estimate to an invoice for your customers: Set up and use estimates.
If you have any other questions, please let me know in the comments below. I'll be here to help. Stay safe!