You would probably need to raise a case. Try this link -
http://www.cic.gc.ca/english/contacts/web-form.asp
If I might ask, what is the error that you are trying to correct. I would be cautious with your approach. Have they asked you for this correction? I believe there was a user here on this discussion board who provided a PCC that he forgot. It seems that the CIC officers never noticed it was missing but when he sent it, it was brought to their attention that it was originally missing and the application was rejected.
Have your ordered your GCMS notes? How certain are you of the error?