To change someone from a Follower to an Administrator, you need to send them an Administrator invitation:
From the Toolbox, tap Followers
Tap "My Administrators"
Tap the plus at the upper right
Enter their email address and tap the green check mark
To change someone from an Administrator to a Follower:
From the Toolbox, tap Followers
Tap "My Administrators"
Tap the red trash can next to the person's name you want to change
This will remove them as an Administrator, but keep them as a Follower