If you mean how do you cancel your payments for the phone, then you can't cancel them. You are still liable for the full cost of the phone even though you have lost it.
Have you checked whether any household insurance policies in your family might cover the cost of the phone?
If you mean that you are paying every month for a goodybag that you no longer require then you need to go here:
Switch off goodybag auto-renew and remove your credit/debit card details.