Welcome to the community!
If you brought your own phone, you would likely have gotten a bring your own phone (BYOP) plan. Those plans do not have any subsidy attached to them and are not available on contract. Did you get a BYOP plan or one with subsidy included (ie Small, Medium, etc)?
Regardless, even if you have a subsidised plan, you should be able to upgrade your plan within the same subsidy level. That is, if you have a Small plan, you should be able to change to a different Small plan without having to pay off the balance of the phone first. Similarly, you should be able to change a Medium plan for a Medium plan, etc.
You should be able to change your plan details via My Account --> Usage & Services --> Mobile --> Change Plan.
Hope this helps 😀