You need to make an online request to withdraw your application. About a week later, you will get an email from CIC advising the following:
A note has been added to your file to advise the responsible office of your request to withdraw your application. Once your application is opened, it should be cancelled and it will be at the responsible office's discretion to grant a refund of the processing fees within 16 weeks.
For information purposes, if the principal applicant withdraws their application, the office will also withdraw the application for the family members included in their application.
Please note that your decision is final and that a new application will require the applicable fees.