Contracts are legally binding agreements. A good, legally reason is needed to get out of it. You also want to make sure you review the contract and that your client contract has a clause built into it that allows you to get out of it under certain circumstances (i.e., health issues, family emergencies, etc.)
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.
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.
O2 end of contract tariff options If you happen to be out of contract with O2 already, it will send you an annual reminder that you're still paying for an old tariff. In these notifications, O2 will offer you a selection of new plans that you could move onto once your fixed-term is finished.