To change the present address, U have to send them a signed fax or a signed letter. They also say that they will not reply you that they have received the updated address.
When I requested for chage of address, I received the following reply.
Please send us a signed fax or a signed letter (see fax number and address below) with the following information:
-your original request to change your mailing address,
-the full name of applicant and original signature,
-the applicant's date of birth,
-the file number [letter followed by a nine-digit number],
-telephone number, including area code,
-fax number, including area code,
-the mailing address, and
-a return fax number and/or e-mail address (if available)
Canadian High Commission, Islamabad
P.O. Box 1042, Islamabad, Pakistan
Fax: +92-51-208-6914
Hope this helps