Welcome to the community!
How were you notified about the overcharge? You might want to be careful with any emails or text messages of this nature. They are often SPAM and scammers trying to get information or money from you. You can view other examples of such SMS SPAM here.
If you had been overcharged on your last payment, the extra monies would be added as a credit on your account. If you have credit balance on your account and you have left Fido, they will send the amount to the mailing address they have on file. They will not send any correspondance requiring you to access links for reimbursement.
**edit** If the message you received was SMS, you can forward the message to 7726 (SPAM) for audit and hopefully it will be blocked**
Hope this helps