Open any of the Office 365 programs and select File from the first tab on the toolbar.
Once that menu opens select Office Account.
Under the Office product information in the middle of the screen you will see four options including Office Updates; Office Insider; About Outlook and What's New.
Select Office Insider to open up the menu and select Change Level.
Select the drop down menu next to Choose your Office Insider level and click on Insider Fast.
Check off the box that you agree with the terms of use for the Office Insider program then click OK.
Once you have opted into the Office Insider Fast Ring you will be returned to the Account information page and you should see the Office Insider area updated and reflecting your enrollment in the Fast Ring.
To get updated to the latest Fast Ring for Office Insiders just click on the Office Update option just above the Office Insider button.
The latest build will now be downloaded and you will be prompted to close the current Office program before the update is installed.
Once you are updated to the latest Fast Ring build you will see that information updated.
If you join the Fast Ring today you should get Version 1609 (Build 7329.1000).
If you join the program later then this build number will be different.