You can promote a user to be a team admin or a channel admin.
To promote a user to a team admin, open your team settings. You can do so by selecting the blue cog next to the team name and the Team Settings will be visible. Select ‘Manage’ next to ‘Admins’. You can then add the user you want to become an admin by selecting the ‘Add a new admin…’ option and choosing to ‘Promote to Admin’ from the list.
To promote a user to a channel admin, first open the list of users in a channel. For instructions on finding this list Viewing Team or Channel Members.
From this list select the user you want to promote. This will show their profile. Toggle the ‘Promote to Admin’ option to set their status as an administrator.