Here is the reply from Fizz customer service. You can try again tomorrow.
I am sorry for the inconvenience. As I can see you subscribed today. You will need to wait 24h before the button will be available under "My plans"/"Manage plan"/"Appointments"/"Reschedule".
Thanks for the understanding!
Have a good one!
It looks like Whizz already answer your question. So try to wait for 24h and see if you can reschedule. If you're still facing trouble rescheduling then contact Fizz directly.
If the chat bubble does not appear, this means that the number of sessions waiting for our answers surpasses our processing capacity. If that happens, send us a private message via Facebook Messenger or Twitter.
Your can also send a message via facebook messanger https://facebook.com/fizzca and click send a message (this is a bit slower)
You can also fill a contact form from your account Once logged in, from the account panel go to My Profile → Contact Form (the subject are limited) and fill out the form and someone will email you back.