What you do is you change it's sleeping time, it will come off, then let it go back to sleep again. You can put it back on or keep it off in the morning when it wakes up again.
hey u know the weird thing is, is that some costumes u buy from the shop u can only wear once, and some ones you can wear over and over. Like when i bought a panda costume and a sheep costume, which was way less cuter that the panda costume my tamagotchi could only wear the panda one once!!