Hello! To be honest, I don't think you can. I'm currently stuck in the same situation as you and I've contacted them and they've offered me exchanges/store credits etc. I've had enough of it all (and they have also left me on read multiple times!). I used PayPal to pay for the goods and have contacted them to help dispute it.
I may try contacting Fair Trading Australia (I live in Aus) to assist as well.

service@kiwisub.com

On the website they direct you to the email above but its useless waste of time because this was the response and despite my taking photos they have not responded. American Express is handling my dispute.
Dear Customer,

For your order COB15517SA

In order to process your return request, please tell us the specific item that you mentioned and hang up it up to take a clear picture to show the problem as part of our documentation so that, we can provide a solution for your problem.

If you have any other concern, please feel free to contact us.