Configuring preferences and options in Outlook.com
The desktop version has a 'slider' which can be clicked on to switch between the original and beta versions of mail. It is in the upper right area of the main page. With the mobile phone version, you need to scroll to the very bottom of all your folders (inbox, sent, drafts, etc) and you will find the same slider there. Took me a while to accidentally discover that, and it made my life so much better, being able to switch off the beta version!