You can't change the password because you are using a single, fixed password:
string Password = "password";
And you can't change that because it is part of your application.
In order to start writing a "change password" form, you first need to start storing users and their passwords somewhere outside your app - a text or binary file, XML, JSON, whatever - or in a database. Until you have decided exactly how you are going to do that, you can't even start to write a "change password" form because you have no idea where or how it is going to be stored!
And do yourself a favour: Never store passwords in clear text - it is a major security risk. There is some information on how to do it here:
Password Storage: How to do it.[
^] but it may be a little too advanced for you at the moment.