From the dashboard, click the "Club Management" tab, followed by "Roster Builder"
Select the "Event Roster" in the dropdown and click "Submit"
Click the "Red X" next to the players name that you are trying to release, then click "OK".
Note: If you do not see a "Red X", its because that roster is locked by the league. You need to contact the league to find out their specific process on releasing a player.