hey, you actually checked this! well, i'll give ya more detailed instructions in case you're super new.
basically, the "mod_overrides" folder goes into the steamapps\common\PAYDAY 2\assets folder, and you just drop the "Prettier GenSec Units" unzipped folder inside of that. pretty simple!
if you have the "Beardlib-less" version, it'll work with vanilla, but if you have the "full" version, then you need to have Beardlib. you can install it from modworkshop, and you need SuperBLT for it to work. 

anyway, if you're still here, considering playing Deep Rock Galactic! it's the game that i quit payday 2 for, since i think it plays a lot better and is more coherently designed. 

i promise if i ever find the original files for this mod, i'll do some more updates to it, but don't hold your breath!