In order to change the Client Admin, you have to first add the new Admin to any of the apprenticeships listed under that Client and select "Admin" as their role.
If the new Client Admin is already in one of the apprenticeships but not as an Admin role, click on Add Member and select Existing User. Enter the user's email address and select "Admin" as the role.
Now that you have the Admin added to at least one of the apprenticeships under that Client, you can go to the Partner Portal and click on the three dots next to the Client to bring up a menu.
Click on "Change Client" and a dropdown list will display where you will see a list of Admins under that Client.
Select the new Client Admin and click the "Change" button.
The old Client Admin will still be an Admin so if you would like to change their permissions or dismiss them from any apprenticeships, you'll need to take those steps after completing this.