Boot up the machine, and after the BIOS screen, hold down the left Shift key. You will then be prompted by a menu that looks something like this:
Hit the down arrow until you select the 2nd entry from the top (the one with the recovery mode in the description) and then hit Enter.
Now you should see this menu:
Using the arrow keys scroll down to root and then hit Enter.
You should now see a root prompt, something like this:
At this stage you should have a read-only filesystem. You have to remount it with write permissions:
mount -o remount,rw /
Now we can set the user’s password with the passwd command. (In this example I will use jorge as the example, you need to substitute whatever the user’s username is):
root@ubuntu:~# passwd jorge Enter new UNIX password: Retype new UNIX password: passwd: password updated successfully root@ubuntu:~#
Type in what you want the new password to be at the prompt. After it’s successful reboot the machine and the user will be able to log in with their new password.