When you're in the store, you will see your two goats near the top of the screen. Tap whichever goat you want to dress up.
Tap a piece of clothing to put it on. If you don't already own the item, you will need to buy it first.
Tap on the Green Check to purchase the item. If you don't want to purchase, tap on the red X.
To take it off, tap it again.