For security reasons, you cannot change the password for any of your email accounts from within the app. To change the password for your any of your email accounts, please access your email via your email provider's Web mail service. For example, if you have a @gmail.com account go to gmail.com. Once you're signed in, you'll be able to change your password. The change password option is usually found under account settings or in your profile.
The next time you use that email account in the app, you'll be prompted to login again with your new password. You can also delete and re-add your email account if you are having trouble updating it in the app.