To make an existing member of your team an admin, all you need to do is toggle 'Team Admin' to ON.
Start by navigating to the 'Manage Team Members' for the team you're wanting to make changes to. Select the pencil icon next to the person you want to make an admin, this will allow you to edit their permissions for the team:
You'll see you can toggle 'Team admin' to ON:
You'll see the member is now an admin of your team:
