How can I cancel my mobile plan?

You can cancel your account from My Plan, however' I will suggest you to chat with the fizz's support. Alos, you need to think if you want to transfer you number, therefore, you cannot close like that. Etc.
The chat https://fizz.ca/en/contact-us
Read a bit here https://fizz.ca/en/faq/unsubscribe
(for the chat . go on the page and wait 15 seconds for the icon to appear at the bottom right)

I suggest you contact fizz on Facebook messenger with this link: https://m.me/fizzcaand see if they could put your plan on hold after you activate it and pay for the first month.
Otherwise, I'll suggest you to compare the plan you want to get with other providers and evaluate whether you may at least "break even" by diluting the amount paid from April to August over, let's say, one year (that is until September 2020). If you are following my reasoning, you will see that it might be worth.

Just a friendly suggestion. Maybe should you wait to find another provider and activate your subscription elsewhere before to close your account with Fizz. Otherwise you won't be able to keep and transfer your actual phone number if you close your Fizz account too early.