In this example I'll change head model of one of my player from youth academy to Messi.

You can find the IDs in FIFA 21 IDs sheet in "Head Models" tab.

Editing player head model through GUI

1. In game go to squad hub and hover the player (W.Henry in my case)
2. In Cheat Table in GUI click on "Players Editor" button and wait until it will load player data
3. To change player head model we need to change Has High Quality Head to YES, Head Class Code to 0, Head Asset ID to ID you want to use. Also, color of the head model can't be changed, so you may need to edit "Skin Color". All these fields are located in "Appearance" tab. For IDs check FIFA 21 IDs sheet. Copy the value from "Head ID" column, put it in "Head Asset ID" in Cheat Table GUI and press "Apply Changes" to save it.
4. Exit and enter squad hub again to see the changes
5. Done

Editing player head model through cheat engine

1. In game go to squad hub and hover the player (W.Henry in my case)
2. In Cheat Engine navigate to "FIFA Database Tables" -> "Players Table" -> "Appearance"
3. To change player head model we need to change hashighqualityhead to YES, headclasscode to 0, headassetid to the ID you want to use. Also, color of the head model can't be changed, so you may need to edit skintonecode You can find the ID in FIFA 21 IDs sheet. Copy the value from "Head ID" column, put it in headassetid.
4. Exit and enter squad hub again to see the changes
5. Done