Hi @C Pradeep
Welcome to our forum!
Based on my testing, there are several relevant settings for the simplified and classic ribbons in Outlook 2021 LTSC:
At the top-right corner, select the Ribbon Display Options icon. This setting collapses/displays all ribbons. If you have hidden the ribbon, you can click the ellipsis (...) to restore the ribbon temporarily. It is possible that the simplified ribbon is restored.
In another case, if you don't have the above settings, you can switch directly through the drop-down arrow in the bottom right corner of the ribbon. When hovering over the arrow, there will be a toggle prompt.
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