The location for accessing Outlook apps is being moved from the bottom to the left. Outlook users on Current Channel have been able to opt-in to try the new experience since March 2022. This will soon become the default experience for navigating Mail, Calendar, People, and other apps (starting with Current Channel version 2207).
Outlook displaying key modules to the left of the folder pane
This update will provide a consistent experience across Outlook, Teams, and Office.com for switching apps and it creates room in the UI for more apps to integrate with Outlook. You can easily launch popular apps like To Do, Yammer, Bookings, Word, Excel, and PowerPoint without leaving Outlook, with more to come. You can also customize which apps you see by right-clicking on your preferred apps to pin them.
The Outlook team recognizes this is a big user experience change and it will take time to get used to the new location for Mail, Calendar, etc. Because this new experience replaces the previous navigation experience and adds new functionality, there isn't an option available for returning to the old navigation experience.