First, make sure you save or back up all the important data you have on your phone. When you change the email you will lose it all. Another good thing to do is to make a list of all the apps that you are using so that you install and configure them again, once the new Microsoft account is used on your phone.
Then, on your Windows Phone go to Settings -> System. There, scroll down to About.
Tap on About and scroll down to the Reset your phone button.
Tap on it and you are asked to confirm that you are OK to ahead and reset the phone and lose all your personal content. If you made the necessary backups, go ahead and tap Yes.
Your Windows Phone restarts and resets itself to the default apps, files and settings. When you start it again, you are welcomed by a wizard, taking you through its initial setup, just like when it was new and you started it for the first time.
Go through the wizard and provide the details of your new Windows Live ID (Microsoft account).