Changing the master admin in QuickBooks Online (QBO) is easy peasy, @m-azam.
When you want to change the master admin, you'll need to make sure you're logged in to QBO as the current master admin. Then, choose the existing employee you want to be the master admin.
However, if the person who has the master admin account is no longer available, send us a request to change the master admin.
I'm adding this article for more details: Change your QuickBooks Online master admin user.
You might also want to check out this article to learn how to manage user access in QBO: Add, delete, or change user access.
Keep in touch if you need any more assistance with this, or there's something else I can do for you. I've got your back. Have a good day.