Copy and paste it in your fm document folder. I already change file name. This file from fm 22 . If it's not working maybe because of different version so you need to do it yourself.
Just play with football manager resource archiver because there's many thing you can mod with that. I learn this when i'm looking to change manager outfit texture.
Maybe you can learn from this
Just use same method to mod your untuck kits. Sorry i'm bad for tutorial.
art.zip