Log into your MyO2
http://www.o2.co.uk/myo2 and go to "Bills"
Scroll down and you will see an option to Manage your details.
You can change your bank account details in there but will need your phone with you to set up a PIN the first time you use it.
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)