You have to know current password to change it
Click on Start and then Control Panel.
Click on the User Accounts and Family Safety link.
Note: If you're viewing the Classic View of Control Panel, you won't see this link. Simply double-click on the User Accounts icon.
Click on the User Accounts link.
In the Make changes to your user account area of the User Accounts window, click the Change your password link.
In the first text box, enter your current password.
In the next two text boxes, enter the password you would like to start using.
Entering the password twice helps to make sure that you typed your new password correctly.
Click the Change password button to confirm your changes.
You can now close the User Accounts window.