change root password
 

how to change the root password without knowing the earlier one.......

One way is to boot up and use the kernel option "init=/bin/bash". This will run the root shell right away instead of init. Then use the passwd command and enter the new password.

Another method is to boot up with a rescue disk, mount your root directory and edit out the root password from either /etc/shadow or /etc/passwd. Then use the passwd command to create a new password.

You could have searched this site. This question has been answered many times.

You could also just boot in single user mode.
This will boot in root without asking for a password and allows you to change your root password with the passwd command.

In Grub:
Edit the boot command and add "single" at the end of the line.

In LiLo:
Boot your system with "linux single"