You can only change it back with a reg edit: But I hope that MS listens and gives us an option to choose within Outlook and revert it back easily for everyone.
1) Open Regedit: In the search box on the taskbar, type regedit, then select Registry Editor (Desktop app) from the results.
2) Navigate to Computer\
HKEY_CURRENT_USER\SOFTWARE\Microsoft\Office\16.0\Common\ExperimentEcs\Overrides\
OR for some people only this folder works
HKEY_CURRENT_USER\SOFTWARE\Microsoft\Office\16.0\Common\ExperimentEcs\outlook\Overrides\ Microsoft.Office.Outlook.Hub.HubBar
3) If this string "Microsoft.Office.Outlook.Hub.HubBar" does not exist, create it by right clicking on Overrides in the left pane and selecting New->String Value
4) Paste "Microsoft.Office.Outlook.Hub.HubBar" in the new entry that appears in the right pane
5) Double-click the new name
6) Enter "False" in the Value data field.
7) Click OK
8)Close and restart Outlook
Originally posted by LD Fix on page 39 on this MS Forum in July https://answers.microsoft.com/en-us/outlook_com/forum/all/how-do-i-move-email-calendar-people-icons-back-to/6dd0ded4-7a9e-4d0a-aa6f-1870e0ef8ac1?page=39