1. Export sculpt nif (make sure it's the same body weight as the npc you wanna replace)
2. Place it where it's needed, I usually put it near the one I want to replace, both copied in a new mod with the same paths

SSE Nif Optimizer here!

1. Select the folder the nifs are in and select just "Heads only"
2. Click on "LE" and run the program

Nif Merger here!

1. Drag and drop your face on the left and the og on the right
2. Check that all head parts are matching in the right tab and activate them (if any are missing you can copy-paste them in later, through any nif editor program)
3. Do NOT click on "merge child nodes" or you will copy colours as well
4. Start with "Merge to target"
5. (optional) if you want to replace hair, it'll warn you vertices are wrong, just tell it to replace them
6. (optional) if replacing hair or eyes, do it a second time selecting hair/eye parts only and also tick "merge child nodes" so you will copy hair colour

SSE Nif Optimizer

1. Tick "SE" and run the program again
2. Profit 🦝