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:
One Player at a Time
League Setup -> Player List -> Click Assign Tees icon next to the player you want to change. It will show all courses that you have entered and you can set the tee for each one.
All Players at Once
League Setup -> Course List -> Click the Assign Tee Box button next to the course you want to set. It will show all players and you can assign tees for each one.
For League With Multiple Courses
League Setup -> Course List -> Click the orange Assign Tees for All Courses button. This will show all players and all courses in one large grid where you can assign for all players and all courses at once.
Assigning By Tee Box Name
If you have multiple courses, you'll see an additional option which allows you to assign a teebox for a player by name all at once. This only works if you name the tee boxes the same for each of your courses. For example, if you had three courses and each had a tee box named "Blue" and "White", you can assign by name by selecting "White" from the dropdown in the column named "Assign By Name". It will find all courses with a teebox named "White" and change it for that player. If a course doesn't have a teebox named "White", it won't change the assignment for that course.