I am unable to update Ubuntu 12.04 LTS due to root password. I have set a password for admin during installing ubuntu but after that I had changed my admin account to standard account. After changed to standard account without any password. On installing any software or updation it asked for root password. On typing root password, it shows wrong password. and unable to authenticate my password. Is my root password which I set during installing ubuntu is changed?
Ubuntu does not have password for root by default so you will never guess it.
What you need to do to fix this is either give root a password which is not recommended practice, or make your self an administrator again.
To do this you will need to boot into a root shell as described here:
Once in you can add yourself to the sudo group with
Replace your_user with your user name.
Next you need to set a password
Now we can set the user's password with the passwd command. (In this example I will use your_user as the example, you need to substitute whatever the user's username is):
Again replace your_user with your user name.
Reboot and you now have an administrator account so should be able to update.
If you don't want your account to be an administrator account you can create a new user instead that is but you will need to do that from recovery mode.