There currently is not a separate login/password for the end user administrator to login and just change the Auto Attendant. Having a multi-tier access control to the GUI is a roadmap item. That said you have two options today
(a) Consider giving them a login/password to the admin screen and training them on how to use the AA.
(b) Check if their AA need can be met by using the Voicemail Greeting, and if yes, then they can change thier voicemail greetings ( or a system voicemail box greeting) on their own without logging into the GUI. The best example that comes to mind was for a Dr. Office, where they had an after hour AA which said something like "The office is currently closed for <name your holiday>. If this is medical emergency, please hang up and dial 911, or to leave a message stay on the line". So at night time, the main AA just sends all incoming caller to the voicemail box directly, where the customized greeting is played out. You can definately make it more elaborate by using more than one system mailbox, and then stringing together voicemails with Auto attendnats and do what you want, without giving out the login/password to the "ADMIN".
I went ahead and flagged the "Correct Answer" on this post to make it more visible and help other members of the community find solutions more easily. If you still need assistance, we would be more than happy to continue working with you on this - just let us know in a reply.