Place the skin textures of one skin in the StreamingAssets folder in the Yandere sim Data folder to use it 
You have a "YandereSimulator_Data" folder in the Yandere simulator game folder right? :>
Open that folder and then there is a StreamingAssets folder. You put the skin you want in the StreamingAssets folder. Not more than one skin though because it will not work with two skins at the same time.
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