You have to be the company admin to be able to modify user's access in QuickBooks Online (QBO). To edit users:
Log in to your QBO account as the master admin.
Click on the Gear icon to be able to select the Manage Users option.
On the Manage users page, look for the user's name and click on Edit under Action to be able to edit the user.
On the other hand, if you're referring to modifying the employee's login for workforce to view their paychecks online, you don't have the option to edit them, only the users are responsible for there access.