Welcome to the community!
In order to migrate your services to pre-paid, you would need to contact customer service. 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 that there is a migration fee for the process. As far as I am aware, that fee is $10 (see here). I understand that link notes being able to do so within the community. However, the situation has now changed since that post. These forums are community-driven and no longer intended as a venue for customer services. There would also be a migration fee to switch back from pre-paid to post-paid should you wish to do so at some point.
It should also be noted that Fido no longer has a By the Minute or By the Day pre-paid offering. Their current offerings are Monthly, Semi-Annually, or Annually (see here). You can find additional information regarding pre-paid service here.
Hope this helps 😀