This one is simple but can be confusing for new users. The important thing to remember is that your Chromebook password is the same as your Google password, and for that reason can be changed from any computer that can access https://myaccount.google.com .
Once logged in at https://myaccount.google.com (using the same gmail address that you use on your Chromebook!), you can click 'Signing in to Google', then 'Password'. This will allow you to change your password.
