Step One:
From your "Dashboard", click on the "Affiliations" tab at the top of your page. Scroll down to "Summary", and click on the number for the players with that affiliation.
Step Two:
Click the check box next to the players that you want to update, then click "Comp Level/Affiliation" under "Bulk Actions".
Step Three:
Select the new competitive level and affiliation, and click "Save".