You can change the password without knowing the existing password. But this trick works only when your PC was logged in.
For doing this all u need is follow the below steps:
step1: Right click on computer and select manage
step2: Go to Local users and Groups option and then click on users. You can see the list of users.
step3: Right click on the user whose password you want to change and then select set password
step4: click on proceed on the pop-up window
step5: Enter the new password and click on Ok.