Download it, extract it (winrar for exmaple) then replace the file name with the orginals. Eg file you downloaded has the helmets as gen_15_tex_high and gen_15_tex_low, you delete (or make a copy just in case) and replace the orginal of that such file.
Just find the game directory (place the game files are stored) then the helmets are stored in, characters > bespokehelmets. Then just follow the above.