First of all, Niko has to have clothes in his closet. If you don't buy it. Provided that both islands are open. Poerxiusi and Modo have both. It is the icon of clothes on the map, and the mouse pointer will show the name of the shop on it. After buying it. I put it on me. If you want to change the wardrobe near your home, press E to enter the changing scene, change clothes around the key, change your hat, eyes, shoes and pants up and down, and usually change a suit later.