To make a user a Head of a Team you need to have Team Management permissions or you must be the account owner. Open the Users section of your account, choose a user and open their User Management screen. In the Teams section you will see a star sign to the left of the user. Click it if you need to make this user a Head of a listed team and choose Set as Head. Note that only business account owners and users who hold Team Management permissions can make a user a head of a team.