Why did Persia change its name to Iran?

Answer 1

Historical name for a region roughly coterminous with modern Iran. The term was used for centuries, chiefly in the West, and originally described a region of southern Iran formerly known as Persis or Parsa. Parsa was the name of an Indo-European nomadic people who migrated into the area c. 1000 BC; the use of the name was gradually extended by the ancient Greeks and other Western peoples to apply to the whole Iranian plateau. The people of Iran have always called their country Iran, and in 1935 the government requested that the name Iran be used instead of Persia.

Britannica.com

Iran has had this name from first. but 2 thousand years ago the famous empire of Iran established , and because the kin is from "pars" tribe , they called the capital pasargad and some forein countries as Greece recognized as 'parse' which later changed to Persia.
the British and Americans mostly in 1935 , Persian/Persia is still a distinction jsut not as widely used internationally.

Answer 2

Persia was the name that the western world had given Iran. Inside Iran, the country was referred to as Aryanam, or "Land of the Aryans." So when in 1935 Reza Shah asked that the country be called "Iran" (derived from Aryanam), it was not a change of name, but being asked to be called by that which they'd always called themselves.