Lenovo, the world's fourth-biggest personal computer firm, said it is replacing its American chief executive with Chairman Yang Yuanqing and bringing back its co-founder to head the board. Mr. Yang took over as CEO from Bill Amelio, who stepped down at the end of his three-year contract. Succeeding Mr. Yang as nonexecutive chairman will be Liu Chuanzhi, a current Lenovo board member who helped start its predecessor company 25 years ago. Lenovo was founded in China but maintains its headquarters in Raleigh, North Carolina.