Go to your O2 account and select 'Membership and Subscriptions' Scroll down to 'Cancel Subscription'
Canceling a Cellular Service Plan To get out of a phone service contract, contact your plan's customer service. You can also try to sell your plan, contact a different carrier for help, or view your contract for loopholes.
Almost all providers require 30 days' notice if you wish to leave.
Contact the business and say you want to cancel the contract because you're still in the cooling-off period. You'll probably need to give them details such as your account reference number - check any documents or emails you have from the company.