Welcome to the community!
You can login to the MY ACCOUNT section on fido.ca, click on the phone number you want to change the plan for. On the next page under MY MOBILE PLAN you will see a yellow button that says CHANGE PLAN, click on that and you can see the list of different plans to choose from.
Keep in mind if you are currently in a contract and you are switching to a different plan tier eg. small -> medium you will have to pay off the current device balance before you can switch plans