So First Step is Boot in to Recovery Mode:
You will see the following screen. Now select "root - Drop to root shell prompt" and press "Ok"
Here is video tutorial
If you forgot your username enter this command to know:
In linux case is sensitive about username and password, If you have upper case than you need to enter same letters.
Mount root with following command:
Now Enter following command to reset your password:
For example (See the picture above):
Now enter your new password and confirm it.