Should a user's current access level restrict them from accessing a page, the user will receive a message advising they do not have sufficient privileges to view that page.
In this case, any superuser of the system (employees with access level 0) can update that employee's access rights within the Admin page.
To update the access level rights of a user:
Go to Admin > View all employees
Select Admin page (Note, this page can only be accessed by employees with access level 0)
Click on Users
On the left-hand side of the page, click on Update user
Select the Username of the relevant employee
Expand the Access rights dropdown and select one of the available options
Click Update user