How do I change the expiry date on one of our donors recurring credit card donations?
Using iATS for recurring contributions means that CiviCRM only holds onto a 'token' that is used to trigger subsequent contributions in the schedule. Which means that the card information all lives over on the iATS servers, and the easy answer is that you can edit it there using the iATS payments administrative interface.

But also note that the next release of the extension (tentatively 1.5.0) in a couple of weeks will allow you to do that editing directly in CiviCRM so it'll be much easier.

And finally - note that earlier versions of the extension will not try and run credit cards that it thinks are expired, based on the expiry date of the card originally used. In other words, in those versions, you'd need to re-enter a new card for a new recurring contribution. That rule was taken out of the last couple of releases (1.4.2? not sure), since expired cards can still work during a recurring series (as long as they are not expired when you first set them up).