Recover password in ubuntu (linux) and OS

RecoverPassword

In unix:

1. Start using grub (if grub doesn't appear, press shift key during booting)
2.- Choose recovery mode (usually the second option).
3.- In the next submenu, choose  "netroot" ("root" in newer versions) --> we will be prompt as superusers without asking for password
4.- You will need write rights (by the default the / is mount as read only)
    mount -rw -o remount /
5.- Change the password
  passwd your user_name
    ej. passwd julian
6.- reboot (sudo reboot) or shut down (shutdown -r now

In OS:

1.- Insert the OS disk 
2.- there is an option that allows you to change the password