user password changing
For a user named koha:

sudo passwd koha
Enter new UNIX password:
Retype new UNIX password:
passwd: password updated successfully

Adding sudo privileges

Not this way: sudo vi /etc/sudoers

# This file MUST be edited with the ‘visudo’ command as root.

This way:

$ su root
# visudo

# User privilege specification
root ALL=(ALL) ALL
koha ALL=(ALL) ALL (Add here, like koha user)