Any Custom League commissioner has the ability to edit the roster of every team in his or her league as long as the team manager has given the commissioner permission to do so. If a manager has set the commissioner control option to "No," you cannot edit their roster, nor will Yahoo! grant you access to do so.
As a manager, you have the opportunity to turn on this feature during registration, or at any time during the season via the Edit Team Info link. When you've set this to "Yes," both you and the commissioner of your league can edit your roster. When you've it set to "No," only you can alter your team.
As a commissioner, you can edit another team's roster by going to your Commissioner Tools page and choosing the "Edit Rosters" option. Once there, select the team you want to alter and the type of transaction from the pull-down menus. You're then presented with options that allow you to make the desired moves.
Since you can only control one team at a time, you must return to this page and follow the same procedure each time you want to control another team.