Here are the steps to change your existing password in Windows 8.
WIN+W to search Settings and search for Users.
Click on "Users", as in:
Go to PC Settings where you will see Users is selected.
Click on the "Change your password" button.
On this screen, you need to provide your current password and new password (twice).
Hit "Next" and you are good to go.
You can also set a picture as your password including touch signals. Check out