To those that have struggled with updating their payment details after the TPG billing change as there is no option to update details, make sure you login to My Account with your mobile number and not your username etc.
I have been helping my inlaws and was logging in with their username and there was no 'Edit Payment Method' option. I logged in using their mobile phone number and their normal password and the option now appears.
Hope this helps somebody as it did my head in lol
