You can find your closet under the customisation menu at the top of the screen. You can remove items from your closet by selecting the items youd like to remove and clicking the Move items button at the bottom of the screen.
Remember that if your Neopet is currently wearing an item you will need to remove it from your pet before you can take it out of your closet!
Special deluxe paint brush set clothing items cannot be removed once they have been added to your closet.