Thanks for dropping by, @pramod-startup-m. I want to make sure that you'll get the answer you expected.
Before I proceed to an absolute solution, may I know what you're trying to accomplish with customising an invoice and changing the primary email ID?
I'll appreciate it when you share more details and screenshots. In the meantime, you can check these articles for more relevant details to your post:
Customise invoices, estimates, and cash memos in QuickBooks Online.
Change your email, user ID, or password for QuickBooks Online.
Feel free to place the details by clicking the Reply button. I and the Community members will help always. Stay safe and have a nice day ahead!