In QuickBooks Online (QBO), you'll have to delete the account first and then add a new one. You can check this article on how many users you can add to your QBO account: How many users can I set up?
Once done, you can follow the steps below to add a user:
Go to the Gear icon, then select Manage users.
Select Add user, then choose the user type you want to create.
Enter your new user's name and email address.
Click Save.
You can also visit this article to learn about the types of users you can add and what they can do in QBO: User types in QuickBooks Online.
If you want to delete an old user in your account. Here's how:
Go to the Gear icon, then select Manage users.
Find the user you want to delete, then click Edit drop-down arrow.
Select Delete twice.
For future reference, you can check these article if you want to change master admin or email address and user ID: