Pretty much what Mike Painter said that it no longer exists.—But here's something of interest regarding this ancient city. . .
On was an ancient and renowned city in Egypt, located a short distance NE of Cairo, on the E bank of the Nile and near the point where the river's waters divide to begin the formation of the Delta region. In Egyptian records the city's name was written as Junu, while Assyro-Babylonian records mention it as Ana or Unu. The Egyptian name is thought to mean "City of the Pillar," perhaps referring to the obelisks (tall, tapering columns topped by a pyramid-shaped point) for which the city was famous; or the name may relate to the sacred stone (called the benben) connected with the worship of the sun-god Ra (Re). The Greeks called the city Heliopolis, meaning "City of the Sun," because it was the chief center of Egyptian sun worship.
On first appears in the Bible record as the city of the priest Potiphera, whose daughter Asenath was given to Joseph as his wife. (Ge 41:45, 50) The name Potiphera itself includes the name of Ra the sun-god.
In course of time the priesthood of On became very wealthy, rivaling the priesthood of Memphis in this respect and being surpassed only by the priesthood of Thebes (Biblical No-amon). Connected with its temple to the sun, a school was operated for training priests and for the teaching of medicine. Greek philosophers and scholars were drawn there to learn the priestly theology, and On became celebrated as a center of Egyptian wisdom.
Hope that'll take you to the spot on the map.
Enjoy!