You cannot activate/bench in the tournament game unless the projected games for one of your players increase during a contest.
This will only occur when the projected games for a roster player you have selected increase after your roster has been submitted. Typically this will happen with Goalies in NHL when they are originally projected to be a backup and are then scheduled as a starter. It may also happen for Pitchers in baseball if a scheduled start is moved forward in their team's rotation.
The situation could resolve itself if a start is removed later in the week. If it doesn't resolve itself and you choose to play the added game, you could be over the salary cap which will mean games later in the week will not count. Those games will be highlighted red in the schedule view.
To make the necessary changes, navigate to your dashboard and select the tournament contest. On the next screen, click on the “schedule” tab, and review your roster schedule to deactivate your desired game.
Review your roster and if you choose to take the added game use the toggle to activate the player. By default, all added games are set to deactivated/benched status.