Lyft grappling with competition and a battered stock price, is tapping a board member as its chief executive, and its two co-founders will step back from managing the company, the ride-sharing company said Monday. David Risher, who had management stints at Amazon.com Inc. and Microsoft Corp. before starting a childhood-reading nonprofit in 2009, will take over from Logan Green, who co-founded Lyft with current President John Zimmer. The pair will retain their seats on the board but not participate in running Lyft day-to-day.