Restart the system
When system boot you will see the GRUB boot loader
In that time press enter then press character “a” a indicate to modify the kernel arguments.
Then type “1” or press “s” for single user mode
For typing one it is indicate it is runlevel one for single user mode.
Then use password command “passwd” to change the root password
After changing the CTR+D for log out
Then restart your system type you new password which one did you create. Then you can login as root to your system.