Iconic sneaker brand Nike is replacing its CEO. Thearon W. Henderson/Getty ImagesNike's new CEO, Elliot Hill, admitted things "haven't been easy" for the brand in a message to staff.Hill, a Nike veteran, will replace John Donahoe as CEO.He said employees need to show "speed and urgency" to turn the sneaker brand around.In his first message to employees, Nike's new CEO acknowledged that the company is facing challenging times.