As creator and admin of a group, you can make other members into admins or remove them.

To manage admins from your group:

1. Go to the group
2. Tap on the three dots from the right upper corner
3. Tap 'Manager Members'
4. Tap on the three dots next to the member you are interested in
5. Select 'Make admin' or 'Remove from admin' as you prefer

If you make a member into an admin, an invite will be sent to that member and they will have to accept to become one.

If, on the other hand, you remove the admin status from a group member, please note the user will still be a member of the group.