How do I change the scheduled installation date after it's been booked?

You can log into your Fizz account to reschedule or cancel your technician visit up until the day of your appointment. Service calls (for repairs), however, are the exception. 

To reschedule your technician appointment:

1. Log into your account and go to Manage.
2. Click on Appointments.
3. Here you will be able to see a complete list of all your past and future appointments.
4. Scroll to the appointment you wish to modify.

• 
  To reschedule an appointment, click on Reschedule.
  • This will open the booking interface. Select a new day and time slot and click Save.
• 
  To cancel an appointment, click on Cancel.
  • On the next screen, confirm that you wish to cancel this appointment by clicking Cancel again.

The Reschedule and Cancel buttons won't be visible in your account within 20 hours of the appointment time, because the tool cannot change the technician's schedule for the day. In this case, you have to contact us to modify your appointment.

To reschedule or cancel on the same day as your appointment, you must do so via a member of our Customer Service team : simply log into your Fizz account and click on the chat bubble in the bottom right corner of this page during our business hours.

You can reschedule your appointment up to three (3) times. If you try to reschedule a fourth time, your subscription will be automatically cancelled, and you’ll have to restart the process all over. 

https://fizz.ca/en/faq/how-do-i-manage-appointment-technician-visit-my-service-address