converting pixel colours of the pathway to white of an hsv image

Please help me with my project. I am not sure how to do as I'm new to Matlab.

http://www.panoramio.com/photo/58624095

This is the picture I'm working with. This is my codes so far. I only need the horizontal pathway to be white. This does NOT include the staircase and the perpendicular sidewalk.

f= imread('Ang Mo Kio Street 53 new.png');

imshow(f)

hsv_f=rgb2hsv(f);