Who renamed the Byzantine capital Istanbul?
It was the local name since the Middle Ages.
There's some debate about the origin of the name. Medieval Greeks referred to the place as "εἰς τὴν Πόλιν" (Is Tim Polin), which meant "To the City." Muslims called it "Islambul," which meant "Plenty of Islam." "Stambul" was the name of a small district right on the Sea of Marmara.
Anyway, from 1703-on it was referred to by Westerners as Constantinople and by locals as Istanbul. When Turkey adopted the Roman alphabet in 1928, their government retired the name Constantinople and asked foreigners to please do the same.