The steps to assign app admin role vary between the three User Interface versions supported in Zoho One. Select the UI version you use from the tabs given below and proceed with the steps that follow.
One Experience UI
Sign in to Zoho One, then click the Admin Panel icon 
on the top-right corner.
Click User Management in the left menu. (You'll by default land on the Users tab). Then, select the user you prefer to promote as App Admin.
Under Applications, click 
. Select Creator in Assign Applications.
Under All Applications, select the custom app you prefer.
Select App Admin in Assign as from the drop-down list, then click Done.
Click Done.
Click Assign. The user is now promoted to App Admin.
Spaces UI
Sign in to Zoho One, then click the Directory Settings icon 
on the top-right corner.
Click User Management in the left menu, then select the user you prefer to promote as App Admin.
Under Applications, click . Select Creator in Assign Application.
Under All Application, select the custom app you prefer.
Select App Admin in Assign as from the drop-down list, then click Done.
Click Done.
Click Assign. The user is now promoted to App Admin. Unified UI
Sign in to Zoho One, then click Directory in the left navigation menu.
Go to Users, then select the user you prefer to promote as App Admin.
Under Applications, click . Select Creator in Assign Application.
Under All Application, select the custom app you prefer.
Select App Admin in Assign as from the drop-down list, then click Done.
Click Done.
Click Assign. The user is now promoted to App Admin.