Follow the info here https://www.o2.co.uk/help/phones-sims-and-devices/our-change-of-mind-policy
You’ll need to package it up how you got it. That means using the original packaging and including any accessory or bundle packs. If you bought your device from an O2 shop you’ll need to return it there. If you ordered it online or over the phone, you’ll need to contact us.
*The Game Is On*