you need to edit the preliminary profile in the Apple development center and add a new device to it .. but you can directly update the profile through a test flight (and not update the profile in the xcode and archive).
After updating the training profile, you should go to your assembly in a test flight, on the Permissions tab (on the left) you can directly update the profile. A test flight will even ask you if you want to send mail to newly added devices (and to old ones that have not yet installed it since the last publication).