There used to be a money exchange center with decent rates at the Yaesu underground shopping center. They have since renovated and I'm not sure if it is still there. That's on the Nihonbashi side of Tokyo station, through the Tokyo Station City area.
If you have cash to exchange, I think you can also go to the major banking outlets, language will be limited of course.
Otherwise, you can also use the post office ATMs as well to withdraw money from your home bank account. (exchange rate is based on your bank in this case)