By The Associated PressActivist investor Carl Icahn is increasing his stake in Herbalife. An SEC filing Thursday showed that the billionaire investor now has a 15.55 percent stake in the dietary supplements company. That is up from an earlier 13.6 percent stake. Herbalife Ltd. and Icahn have agreed that he can raise his stake in the company to as much as 25 percent and add two new members of his choice to the company's board. The investor has been in a very public dispute with hedge fund manager William Ackman over the company. Icahn has backed Herbalife, but Ackman has a negative bet on the company, shorting its stock, and has accused it of being a pyramid scheme. Herbalife shares increased nearly 2 percent to $41.72 in after-hours trading.