Good morning Nnassia & welcome to the Community.
Here's how you can change your SIM card:
1. Log in to My Account;
2. Click on Change your SIM under the My Fido Service section;
3. In the dropdown menu, select the Fido number to whick you need to update the SIM card;
4. Enter the new SIM card number in the required field;
5. Select Next.
Hope this helps. Cheers!