To trade a low rated player for a player who's rating is much higher, first turn off the salary cap. Select the player you want to trade for, go to "Edit Player", and change his position to one that he does not normally play. For example, if he is a wide receiver, then change his position to a quarterback. You will now be able to trade your lowest rated player for him. After you make the trade go back to "Edit Player" and change his position back to his original position.