Lowes and Home Depot both sell concrete cleaner along side their concrete paint products. It works well but so does a gallon of muratic acid.

Do NOT breathe the fumes. Open doors, windows and or rent a huge fan if neccessary. Not nice stuff but it WILL clean the surface sooo nicely.

If the rest of the concrete is aged the cleaned part will stand out so if that is the case get more cleaner. I have done two garages this way, no problem (except the fumes!!)