How to activate sim card in excisting number?

Do you want to transfer the number from another provider?
https://fizz.ca/en/faq/activation-how-do-i-transfer-my-current-phone-number-to-fizz

Or you want to change the sim?
My plans > Manage > Change SIM card

If you are a new customer, remember to use a referral code.
https://community.fizz.ca/questions/2198337-place-referral-codes

Yes, you can use mine