Unfortunately, there isn't a way to fully revert to the "old style" Outlook in recent versions of Microsoft 365 or Office. Microsoft has been phasing out the older interface in favor of the new design.
You may opt for Use Compact view/Use the classic ribbon/Disable Focused Inbox/Use the folder list instead of Simplified view/Change the Reading Pane option etc.
These changes can make Outlook feel more like its previous versions, but they won't completely restore the old interface.
See this thread also: