For current members of the EAC, head over to the EAC testing site and login to your account. You should have received an email already with more information and links.
If you're not a member yet, what are you waiting for?? It's free to join and it gives you the opportunity to test your data and your workflows on a pre-release version of the upcoming ArcGIS Online Update. It also gives you the ability to see and play with some of the new updates before anyone else.
To sign up, head over to the EAC website. Once you sign up, you should receive an email within a day or so with information on how to log in and get started. We look forward to seeing you there!