The Outlook Desktop application for MacOS now has to different interfaces available for users, this article will walk you through switching between these two.
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Switch to New Outlook
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At the top of the Outlook window, locate the Legacy Outlook switch and select it to turn it off.

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Outlook will now switch to the New Outlook edition, this could take a few minutes.
Revert to Legacy Outlook
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With Outlook open, locate the Help tab in the menu bar and select it.
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Then locate Revert to Legacy Outlook and select it.

Additional Support
Further information on New and Legacy Outlook for Mac can be
found here.