You can hide/minimise your Ribbon in Outlook by using the Ribbon Display Options Button. 

Click on the button and choose how you want your Ribbon to show in Outlook.

To undo, just click on the option you selected again.

If your Ribbon is hidden, click on the three dots beside the Ribbon Display Options Button to expand/show your Ribbon.

Alternatively, you can switch between the Simplified Ribbon and the Classic Ribbon by simply clicking on the arrow button at the bottom right of the Ribbon.

The screenshot below shows the Simplified Ribbon. The Classic Ribbon is the default Ribbon in Outlook.