Go to the page, select 'players' and then sort by what you are looking for, in this case defenders. Find the names on the list that comes up, click the little green '+' sign next to the name, a screen will come up asking you to drop a player to add the new player, put a check in the box of the player you are dumping and clcik the continue. It will ask again if you are sure, click continue, it's done, you have the new player on the roster and the old player is dumped.