New major feature: Opt-in toggle to try the early version of the new Outlook on the web

Microsoft is releasing an opt-in toggle for users to be able to try an early version of the new Outlook on the web as Microsoft gather feedback and keep improving the experience. Users can opt-in/out as often as they want.

Microsoft begin rolling out the opt-in toggle on September 10, 2018, to Targeted Release organizations initially and then to all Outlook on the web users.

[How does this affect me?]

Starting on September 10, 2018, users will be able to opt-in to try an early version of the new Outlook on the web. Users can try it by clicking on the "Try the new Outlook" toggle on the top right of their inbox, and they can opt-out and go into the classic experience with the same toggle. Users can opt-in and opt-out as often as they want.
You can learn more about the early version of the new Outlook on the web by clicking here.

For a list of features that won't be available in the early version of the new Outlook on the web that’s releasing on September 10, 2018, please reference this post.
Microsoft be gradually rolling out this toggle to Targeted Release organizations on September 10, 2018, and it will stay in Targeted Release for at least 30 days before Microsoft begin the rollout to remaining organizations. Microsoft anticipate rollout completion worldwide by January 1, 2019. 

[What do I need to do to prepare for this change?]

This feature has administrative controls to enable and disable. Toggle availability is on-by-default but can be disabled by using PowerShell. If you would like to disable the toggle, please follow the instructions in Additional Information below.

https://techcommunity.microsoft.com/t5/Outlook-Blog/An-early-version-of-the-new-Outlook-on-the-web-will-be-available/ba-p/225338