Welcome to the community!
If you have a grandfathered plan that gives you a phone subsidy and want to pay off the balance and change to a new plan you can do so online. Just log in to fido.ca select the number you want to change and then click on change plan you will get a warning that you have to pay off a balance and you can then just select the new plan and continue the process.
If you want to pay off the balance and keep the plan or cancel the line you will need to contact customer service.
You can contact customer service through any of the available methods listed here.