CBSubs has no functionality to update payment sources on file. Once it does that functionality will be implemented in the Stripe gateway.

Looks like the Stripe dashboard only lets you edit/add Cards and Bank Accounts. I'm guessing SEPA is Source and not Bank Account.

Your only option then would be for the user to cancel their subscription and resubscribe with their new billing details.

That's your only option, sorry. CBSubs does not have the functionality to update payment details on file. We've a pending feature ticket to implement support for that in a future release.

When CBSubs was originally developed the focus was PayPal compatibility. PayPal doesn't need functionality like updating payment details on file since you'd just login to your PayPal account. CBSubs eventually evolved to be much larger in scope than originally designed. Thus some features you might expect just aren't there yet. The big problem this poses is that it was built from the ground up to be designed this way. So we've A LOT to rewrite to implement many of the features people have been asking for.

What's worse is because CBSubs manages a lot of peoples financial streams, including ours, everything must be implemented extremely carefully as we can't risk breaking the core functionality. So implementing major features like this that were never planned for from the beginning is very complicated.

With that said it doesn't really solve your problem, but I hope it at least explains why things work the way they do.