Changing the account password in Windows 8
Press the Windows key to display the start screen interface if you are not already there
Enter Control Panel and select the result with a click or tap on it
Click on the User Accounts and Family Safety category and there on user accounts.
Select Make changes to my account in PC settings. This loads the full screen control panel of Windows 8.
Locate the change your password option and click on it
Enter the current password and twice the new password
We are not done yet as it is necessary to configure the auto sign in again. I have detailed how that is done here. Here is a quick summary of what you need to do:
Press the Windows key.
Enter netplwiz and hit enter.
Select the user account you want to configure auto log in for and uncheck the "users must enter a user name and password to use this computer" box.