You can customize the Simplified Ribbon by pinning and unpinning the commands you use most.
Pin a button to the Simplified Ribbon
By default, the Simplified Ribbon shows you the most common buttons for whatever task you're performing in Outlook. Buttons not shown on the Simplified Ribbon can either be found by clicking the drop-down arrow next to another button or by clicking the three dots 
You can pin any button you want to the Simplified Ribbon so it's always visible.
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Locate the button you want to pin to the Simplified Ribbon. For example, to pin the Reply with Meeting button, click the three dots.
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Right-click the Meeting option in the Respond group and select Pin to Ribbon.
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The Reply with Meeting button will appear on the Simplified Ribbon and will disappear from the overflow items.
Remove buttons from the Simplified Ribbon
There are two ways you can remove a button from the Simplified Ribbon.
If you pinned a button to the ribbon, you can right-click that button and select Pin to Ribbon to uncheck that option. The button will return to the overflow area.
If you want to remove one of the default buttons from the Simplified Ribbon, right-click the button and select Remove from Ribbon. The button will be moved to the overflow area.
