Reset password using Grub
Restart your computer, and hold down the Shift during bootup to get into the grub menu.
If you have a dual-boot machine and you choose at boot time which operating system to boot into, the grub menu should appear without the need to hold down the Shift key.
If you are unable to get into the Grub boot menu, and therefore cannot choose to boot into recovery mode, you can use a live CD to reset your user password.
Press the down arrow on your keyboard to highlight the line that ends with the words ‘recovery mode’, then press Enter.
Your computer will now begin the boot process. After a few moments, a Recovery Menu will appear. Use your down arrow key to highlight root and press Enter.
At the # symbol, type:
mount -rw -o remount /
to remount / as read/write.
where username is the username of the account you’re changing the password for.
You will be prompted to enter a new UNIX password, and to confirm the new password.
And finally type:
After you successfully log in, you will not be able to access your keyring (since you don’t remember the old password). This means that all your saved passwords for wireless networks, jabber accounts, etc. will no longer be accessible. You will need to delete the old keyring and start a new one.
Get Rid of the Keyring Manager
This will delete all your saved passwords for wireless networks, instant messaging accounts, etc. Only do this if you can’t remember the password you used for your keyring.
Go to your Home folder by typing ‘home’ in the Activities overview.
Press Ctrl+h (or click View ▸ Show Hidden Files.)
Double click on the folder .gnome2
Double click on the folder called keyrings.
Delete any files you find in the keyrings folder.
Restart the computer.
After you restart and log in you will be asked to enter your wireless networks password.