If you simply want to change your login password for your Chromebook, you just have to update the password associated with the Google account that you use to log in. This is best done from the Chromebook you use regularly. This method takes presumes that you know your current Google password. To begin, open up a New Tab Page on your Chromebook. If you have a custom New Tab Page, you can just go to Google or any of your connected Google services such as Gmail or Google Drive. At the top-right corner, you should see your profile picture. Click the photo and select “Manage your Google account.” It will be the one directly below your photo, not the one at the bottom of the dropdown.
More on this at Chrome Unboxed