on 17-10-2019 17:38
See change of mind policy here https://www.o2.co.uk/help/phones-sims-and-devices/our-change-of-mind-policy
OnePlus 6 (O2 & Sfr), Z3 Tablet (Three UK), iPhone 8+ (EE)
on 17-10-2019 18:36
If you upgraded online or over the phone then return it as per the link. If you upgraded in store then it has to be returned instore.