How does a Team Admin change the Join Code for their team?

A Team Admin can change their Join Code by by selecting "Settings" from the Team Admin Tools menu. 

Update the Join Code to something unique but easy to remember and click “Update Join Code”. Once the join code has been updated, the previous Join Code will no longer function (though players who joined using that code will remain in your team). 

Distribute the new Join Code to players who wish to join the team.