Hi Gnusha-nel,
Welcome to the Forum.
Are you referring to the Restore down icon? (i.e. Minimize | Restore down | Maximize)
If you are then turning off Tablet mode ought to allow you to use the Restore down feature. In tablet mode, you can only minimize or maximize a window.
You can click the Notification > Tablet mode to turn it off.
Also, Start > Settings > System > Tablet mode then under When I sign in select Use appropriate mode for my hardware (Note this setting change will only show up when you next sign in).