In Ubuntu's default setup the root account is not active. Things are done with sudo.
If you still have access to your standard account you should be able to change the password of any other account by running
sudo passwd <account-name>
where <account-name> needs to be replaced with... well... the account name.
Note that it will first ask you for your own password. All of this is assuming that "Backup" actually is a user account! (I am saying this because it is not obvious to me why you would have a "backup" account to begin with, so I am assuming you might be referring to something else than a user account here. If that is the case you please clarify)