Download the helmet, extract it. Make a backup of all the files you're going to replace. Find your game directory, once your into it look for the file marked 'characters', head into that then open up the file marked 'bespokehelmets', then simply replace your download files with the corrosponding file in bespokehelmets.
Eg: F12012 > characters >bespokehelmets > (eg the helmet you downloaded is called gen_01_tex_high, gen_01_tex_low), make a copy of the original as a backup, then replace them with your new helmet.
You can always rename the helmet you just downloaded so it can replace any one of the original helmets, eg you've just downloaded gen_06_tex_high, gen_06_tex_low but you don't want to replace that file, just rename it to any other and then replace it.