Istanbul is the modern name of the city called Constantinople during Ezio's story in Assassin's Creed: Revelations.
The game is set mainly in Constantinople in the early sixteenth century. Ezio travels there while pursuing Altaïr's legacy and searching for keys connected to the Masyaf fortress. The city is divided among districts and communities, including Byzantine and Ottoman factions.
Constantinople was the capital of the Byzantine Empire until its conquest by the Ottoman Empire in 1453. During the period depicted in Revelations, it was the Ottoman capital and a major center of trade, politics, and scholarship.
The question can cause a historical naming mix-up: the game uses Constantinople because that was the period's common name, while Istanbul is the city's modern official name. Alexandria is central to Assassin's Creed Origins, not Revelations.