Welcome to the community!
Are you referring to changing from a post-paid plan to a pre-paid plan? If so, it's possible to migrate from a post-paid to a pre-paid account. As far as I am aware, the migration can only be done by contacting customer service to have them switch your plan types (see here). In addition, they can also be contacted via Live Chat, Facebook, or Twitter. Those methods can be accessed via the contact page posted above.
You should note there would be a migration fee required which would be applied to your next post-paid bill.
Have you had a chance to familiarise yourself with Fido's pre-paid services? You should note that Fido no longer has a traditional pay-per-use (ie By the Minute) plan available anymore. You would need to select a monthly pre-paid plan (see here). You should also note that the top-up denominations do expire depending on their value (see here and here).
Hope this helps 😀