Tools like Vortex or Mod Organizer 2 are highly recommended. They allow you to "overwrite" previous versions of Kirstin without deleting original game files.
Only download from established modding communities. Avoid "Direct Download" buttons on obscure blogs that ask you to bypass your antivirus. War Demon Kirstin Download -v1.11- -Latest Vers... Fix
Ensure that the "War Demon Kirstin - Fix.esp" is placed toward the bottom of your load order to ensure its textures take priority over generic NPCs. Tools like Vortex or Mod Organizer 2 are highly recommended