In the very beginning, Microsoft used multiple accounts to handle multiple services like, to manage hotmail different password and username was required and to access Windows live mail another different email id and password was used. like this, there were many password and usernames that were very difficult for Microsoft and the users to manage their different accounts. So Microsoft decided to combine all the accounts and name it Microsoft account live. Know all the accounts can be managed from account.live.com. you can change any Microsoft password from the account visiting account live portal.
The process to change Microsoft account password is quite simple. You can follow the process used to reset Outlook password here is the link for the detailed steps.