Only a Team Admin can promote a Team Member to Admin. The initial Team Admin is the user who first created the Team.
To promote a Team Member to Admin, navigate to the Team's Projects page, and click on Team Settings
Next, click on General
Here you can see the option to "Add new Admin". Click on this and a window will open allowing you to promote a Team Member to Team Admin
To see all benefits of upgrading your Project to Premium, click here.