Just log into your My O2
http://www.o2.co.uk/myo2 and select the option to "bring in other products". Follow the instructions on the subsequent web pages.
I have no affiliation whatsoever with O2 or any subsidiary companies. Comments posted are entirely of my own opinion.
This is not Customer Service so we are unable to help with account specific issues.Currently using:
Pixel 7a (O2 & Lyca), One Plus 6 (Sfr), iPhone 12 Pro Max (Vodafone)