If you have not enabled Cortana.............
left click at the Search bar > left side, 3rd icon is the Gear ( Settings ) icon > click it >
under Search online and include web results.................... turn it to Off.
If you have Cortana enabled....................
left click at the Search box > left side, the 3rd icon is Notebook > click it > click Settings >
under Cortana can give you suggestions etc etc .................. turn it to Off.
under Search online and include web results.......................... turn it to Off.
That's it.
You will see that the search box will say Search Windows.