Welcome to the community!
Firstly, to clarify, are you wishing to change your phone number to one in the same Province as your current phone number? If you are changing Provinces, you might be required to change your plan to one available in your new Province. If that is the case, you would likely need to contact customer service for the change.
On the other hand, if you are wanting to change your number to a new one available in your same Province, you should be able to do so via your My Account --> Usage & Services --> Mobile. If you do not initially see the Usage & Services tab, you might need to first click on Profile & Settings or Billings tab. Once on the Mobile page, the option to change your phone number is near the bottom under Quick Actions.
You should note that a number change is not reversible. In addition, here are some other things to know about changing your phone number.
Hope this helps 😀