Assuming he is a signed player of yours ..
Click the players name .. it should open a pop-up
Click "View" upper right corner .. takes you to the player page.
You should see a maroon box that says "Change #/Pos" .. as long as you aren't trying to move a punter to DT (Although they may make a good DT given strategy .. lol). You should be able to switch him to the desired position.