Unfortunately, we are not able to process exchanges for online orders. You can certainly return your item/s in store (don't forget to create a QR code here) and purchase another size/colour whilst you're there, stock depending.
You can also select to get your refund to a gift card* when returning your order, so that you get your refund quicker and can purchase the other item (stock depending).
*This option is only available if you paid with Credit/Debit card, Apple Pay or Google Pay.