If you need to make changes to your login credentials, you need to do so on the AAAS Identity Provider system. (If you can't remember your password, you can request a new password.)
Log into myprofile.aaas.org. From there you can change your Username/AAAS ID from the "Edit My Contact Information" tab.
NOTE: The Science & Technology Policy Fellowships (STPF) program maintains separate profile information on you, (aside from your AAAS Username). Please be sure to update your FellowsCentral profile with any changes to your contact information, employment, education, etc.