This tutorial breaks down the process of downloading and exporting the metahuman face and body textures, then adding detail and texture in Substance Painter, then bringing all back to Unreal Engine and reapplying the updated textures to the Metahuman in Unreal Engine 5.6
https://dev.epicgames.com/community/learning/tutorials/oWxG/unreal-engine-create-custom-skin-textures-for-metahumans