Truworths Vaal Mall won’t exchange your items!

Vaal Mall Truworths refuse to exchange. They just refuse to help me exchange something I got as a gift. It still has its tags on and in spotless condition. I just want to change it for a bigger size, but they won’t do it because I Dont have the slip. I don’t want a refund. I just want to change the size and I don’t have the slip because I got it as a gift.