Change the return date flying back from Hong Kong to Vancouver BC

I am going to join a 15 days China tour in October and will return back to Vancouver on November 21, 2011. My reservation number is CA/NX98KQ. I have to stay in China to see a few doctors for my medical check up. I might have to extend my return date for another two or three weeks. Is possible to change the return date? Or any penalty I have to pay? If is possible, how many days ahead I have to contact your office? Thanks Very Much............ Kenny Chan