Updated on the 17/07/20: Please this this tutorial on Direct Debits on the new online account I ended up calling them to change bank details. I was assured the full feature set is gradually being added to the new website. Thanks for the link btw.
Using https://smartpaym.ovoenergy.com/ I see 'This is a new online experience – find out more and give us feedback.' at the top of the screen. There is no 'my profile' tab, there is an account tab that leads to profile, but it does not have any way to change bank details.