Pekah son of Remaliah was Pekahiah's chief officer, but he made plans to kill the king. So he and 50 men from Gilead broke into the strongest part of the palace in Samaria and murdered Pekahiah, together with Argob and Arieh. Pekah then became king.
Everything else Hezekiah did while he was king, including his brave deeds and how he made the upper pool and tunnel bring water into Jerusalem, is written in The History of the Kings of Judah.