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All the options to cancel your contract are in the guide there @Harmeet.
If you're cancelling your contract and want to take your number, you can request a PAC to give to your new provider. If you don't want to take your number, you can request a STAC and give that to the new provider. Once they process either of these, your account with O2 will be closed.
If you're not switching to another provider and want to cancel, you need to contact O2 which can be done through the social media links provided by @Enlli or by calling 202
