For this you need to be logged in to computer from any user account (means you are either member of Administrators or Users Group).
Now, follow the step by step instruction: –
1. Go to RUN(Press window key + R) and type control userpasswords2
2.Select the user you want to change the password.
3. Click on Reset Password Tab (reset tab will only be available if the particular account have password) And Reset the Password.
YOU ARE DONE :D.!!!
NOTE: – Above trick will not work if you are a user of Guests Group.