It will vary depending on how (where) you brought your device @Tame .
If you got it from a shop, you need to take it back there.
If brought online/via phone then you need to start the return the same way.
See o2 help pages below:
https://www.o2.co.uk/help/device-and-sim-support/returning-your-device
Please note, this is not customer services and we cannot access your account. Do not publish personal details (email, phone number, bank account).
Link to our guide on how to contact them can be found here
