You can change US$ at the register inside Cafeteria Paco's at the end of El Conde by Parque de Independencia. If you walk a few blocks down El Conde, you can go to the casa de cambio inside Pollo Rey (inside on the right side) and they may give a better rate. There is also a place called Frank's nearby which some here use, and say gives a good exchange rate also.
DO NOT change money on the street if someone offers. EVER.