Login to webmail by navigating to apps.rackspace.com
Click on the menu in the top right hand corner
Click "Change Password"
Type your current password and then provide your new password twice and click "Save"
You'll now need to update your password inside of all email programs that you use to manage this email account. Outlook, Gmail and your cellphone (Iphone or Android) would be examples of where you need to change this password.