If you have the contributions permission on the group, you can update an activist's card or billing information for a recurring donation or cancel it.
Go to the Start Organizing menu and click 'Activists.' From there, select your group from the drop-down menu, and search for the activist by email, name, or address. Click the 'View/Edit Record' button to view the activist record.
Click the "Edit Subscription" button in the recurring donations section to go to the update your card page. There you can enter in a new card and billing address for the activist to update the one used on that recurring donation, or cancel the recurring donation using the buttons in the lower left.
If the donation is active, the new card will be used on the next payment. If the donation is failing or cancelled, the card will be charged immediately and the recurring donation set back to active, to recur in the future.