If your course has more than 1 tee box, or if you have multiple courses, you can assign a player's tee box a few different ways:
League Setup -> Course List -> Click Assign Tee Box next to the course to change.
League Setup -> Player List -> Click the icon in the "Assign Tees" column next to the player you want to change
League Setup -> Course List -> Click the orange button "Assign Tees for All Courses"