How you can change any user password without having to know the existing password?
First logged onto the computer to an account that has administrative rights, then click on Start>Run>and type in CMD and press OK.
Type in net user then the name of the account then * and press enter. For example: net user administrator * or net user "jhon" *. Put the name in quotes if it contains spaces.
Here it should simply ask for a new password not old password. Type your new password and once more to confirm it. If you get the message that the command succeeded successfully then log off your system and check new password.