I want to know an alternative procedure to the command "passwd" to change the administrator password of my system Linux UBUNTU
If you're able to use "sudo", well, that's it ... you can modify/create a user to have the user id (uid) set to root, along with proper gid.
You can also erase the password from root by hand-editing /etc/shadow, or by using passwd to change it.
If you don't have access to sudo, reboot the machine, and, in grub highlight a entry, type "e" and highlight the kernel line, type "e" to edit it and add
init=/bin/bash
once booted:
mount -o remount,rw /
And then you have proper root access to do what I first said.
you can later sync and do a proper shutdown ...
Regards.