Nikkei opened slightly higher Monday morning amid a weaker yen and climbing Chinese manufacturing data. At 9:15 a.m., the 225-issue Nikkei Stock Average increased 30. 54 points, or 0.23 percent, from Friday at 13,419.40. The broader Topix index of all First Section issues on the Tokyo Stock Exchange was up 2.44 points, or 0.22 percent, at 1,108. 49.