Where is the Update billing url?
If they get the email receipts or have the initial email that notifies it is recurring it has the URL.

This is what I see for WP:

https://example.org/?page=CiviCRM&amp;q=civicrm%2Fcontribute%2Fupdatebilling&amp;reset=1&amp;crid=XX&amp;cs=......

crid is the in the civicrm_contribution_recur table

Not sure what CS string is.