You can cancel your SIM service directly through the My Account Portal.
If you are porting out your number, do not cancel your SIM service. Instead, engage your new provider and ask them to initiate the number port.
Once the port has been completed, the SIM service will automatically be cancelled.
To cancel your SIM service, login to the My Account Portal and select SIM Cards on the left menu.
Locate the SIM service you wish to cancel, then from the Actions dropdown choose Cancel Service. You will be asked to confirm the cancellation before it is submitted.