I want to change my plan of my internet but I do not see the adjust plan option at all on my account.
I'd suggest you chat with Fizz to have a clear and prompt answer.
ALTERNATIVELY, try leaving a message on Facebook Messenger (https://www.facebook.com/fizzca), iMessage or Twitter private. The advantages of Fizz tool are the management of waiting times and the ability to print (e.g. PDF) a copy of the conversation for your records. Sometimes you can even leave a comment about your satisfaction with the conversation.
The WhatsApp way has been described in "How to contact Fizz on WhatsApp?" at https://forum.fizz.ca/en/discussion/comment/6568069
as you see in the picture i shared, only see 3 options
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