Short answers, pulled from the story.
Regent Prince Paul was the official deposed during the Yugoslav coup d'état on the 27th of March 1941. He was replaced by King Peter II after senior officers from the air force and Royal Guard seized control of Belgrade.
The Yugoslav coup d'état took place at 2:15 am on the 27th of March 1941. The operation involved VVKJ officers in Zemun and Royal Guard officers in nearby Belgrade who assumed control of critical buildings within hours.
Adolf Hitler ordered an invasion of Yugoslavia because he viewed the regime change as a hostile act against Germany. Führer Directive 25 issued on the same day declared Yugoslavia a hostile state requiring immediate crushing of its government.
The Yugoslav coup d'état delayed Operation Barbarossa by approximately four weeks due to the need to defeat Yugoslavia first. This delay is considered an important indirect factor contributing to the eventual Axis defeat in the Soviet Union during winter 1941.