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.
you would need to log into that account and delete recurring payments
alternatively if you cant get into that account then you would have to contact an agent
they are usually pretty quick but remember an agent can take up to 24 hours to reply
go to goodybags and payments bluff your way through to other information and explain your problem with as much information as you can an agent will do their best to sort it out for you
for replies from agents
if it a payment for a loan then you cant cancel it sorry