Easiest way to do this is copy Sarah's existing face onto the player character, as some of the facial options are identical. Then change the face, export the face to Sarah and then recover your original face.
1) Save your game before starting so you can revert if needed.
2) Save your existing face. Find an NPC vendor and get close to them in first person view. Open the command console with ~ (default key). Click on the NPC and her ID "NPC_NPCName xxxxxxxx" should show. You then type CAPP 14 which will replace her face with your character's. IMPORTANT: write down the ID number, whatever xxxxxxxx is in the name. You will need this later to recover your face for your character.
3) Import Sarah's face onto player. Go into third person view, open console and click on player and your ID NPC_" (00000014) will appear (you may need to change angle or zoom in, best bet is side view)
Type CAPP 5986. Exit consol for Sarah's face to replace yours.
4) Open consol again, type SLM player 2, exit consol and use the images provide to alter Sarah's appearance
5) Transfer new Sarah look to Sarah. Get close to Sarah in first person view and open the console and click on Sarah Morgan. After clicking on her, her ID "NPC_ 'SarahMorganREF' (00005986)" should appear. You then type CAPP 14 which will replace her face with your character's.
6) Retrieve your original face. Open consol, click on player, type 'capp xxxxxxxx' (xxxxxxxx is whatever number you wrote down in 2) above.
7) Save game. Sarah Morgan's new appearance is permanently locked in.
The texture for Sarah is enclosed in the file
(Drop the Data Folder enclosed in the file into Users\XXXXX\Documents\My Games\Starfield directory)
RECOMMENDED MODS
Eyebrows Redone
The Eyes of Beauty - Starfield Edition
Saved Game
Lvl1 character named Sarah. Saved right after character creation. Background is Diplomat, traits are Alien DNA, Task Master & Terra
Firma.
Place the save file in
Users\XXXXX\Documents\My Games\Starfield\Saves
IMPORTANT IF USING SLM 14 COMMAND (Would be ~ by default on a normal keyboard and then type SLM 14) This
command allows you to change everything just as in the first character creation including personnel record, background and traits.
Use the Personnel Record #1 which will be a male. You then have to change him into a female in the BODY tab section.