General Motors co-CEO Dan Akerson will step down next month, and be replaced by global product development chief Mary Barra, who will become the first woman to lead a global automaker. The company said on Tuesday that Akerson, who is also the chairman, will leave on January 15, moving forward his planned departure by at least six months. He said he had planned to leave in mid- to late-2014, but accelerated his exit after learning about two months ago that his wife had an advanced stage of cancer.