Thanks bigslick202, this clue worked for me, on a Galaxy S6 where I still had access to the phone using a fingerprint, but had forgotten the backup password.  
Short answer: Backup password is stored in Fingerprint app data cache.  So just clear the Fingerprint app's data cache, and you'll be prompted for a new backup password. 
Long answer, what I did was 
-  Go to the Application Manager\Fingerprint app
 
-  'Force Stop' and 'Clear Data'
 
-  Reboot phone
 
-  Unlock w/ fingerprint
 
-  Go to 'Lock screen and security\Screen lock type' (which should be set to 'Fingerprints')
 
-  Use fingerprint to get past the next screen, and 
 
-  Viola, the next sceeen prompts for a NEW backup password
 
-  Now log that password down somewhere in your cloud storage!