It changes automatically depending on the map, you unlock them by completing "One shot, one Kill I, II and III" challenge.
a sewing machine.
Your ghillie suit changes according to the type of condition the map is. For instance if it is a map set in a desert area like Rust or Dome, you would have a ghillie suit with sandy colors and desert camoflauge. Likewise if it was an urban map you would have colors and tones which are similar to the colors of the buildings around the map. This is mainly to help you remain unseen to the untrained eye and helps greatly for stealth and sniping on maps like Rundown, Fallen, Underpass, Array, Outpost, Downturn and Aground.