How to change user permissions from an Admin to Portal User

To change a current Admin user to a Portal User, go to Settings > Basic > Users, and select the edit button to the right of the user.

Next, go to User Access Permissions and un-check both "Is Work Order Management Admin" and "Has Access to Work Order Management" boxes as shown below. Then you will have the ability to check the "Work Order Portal User" (see orange arrow) box and SAVE.