It is so called because the Nile flows north, so it is further upstream and of higher elevation in relation to Lower Nubia.
The extension of Upper Nubia is rather ill-defined and depends on the researchers’ approach.
Geographically speaking Upper Nubia designs the area between the Second and the Sixth cataracts of the Nile.
[1] Physiographic subdivisions of Upper Nubia alongside the Nile cataracts:[2] Politically speaking Upper Nubia falls in present northern and central Sudan stretching from the Egyptian border south to present-day Khartoum at the confluence of White Nile and Blue Nile.
[1] Connecting the Mediterranean world with inner Africa, Upper Nubia was crisscrossed by important trade routes and has been the cradle of diverse cultures.