Normally you would make someone a team manager when you add them to the team. If you are unsure of how to do this, see the article: How do I add members to a team?
If you have already added members to the team and need to make one a team manager, you can do that from within the team list.
NOTE: a team manager must register online in order to update match information on the website.
From Competitions, select Team Setup.
Click the name of your team to open the team record. Click on the Team Members tab.
Click the Enable Member as Team Manager icon.
Click Yes to confirm.