pls help me You went shopping and bought something you were unhappy with .you went back to the shop but the assistance would not exchange it or give you are ref

Good evening/ good afternoon I would like to Exchange this item that does not fit to my liking. I have tried to exchange it but the shop assistant would not let me exchange it or give me a refund. I find this behavior completely unacceptable and unprofessional As the item is still completely new.