Returns must be made within only 48 hours of receipt, while exchanges are allowed for 5 days. The item has to be unused, in its original state and packaging, and accompanied by every accessory, any gifts included and the invoice; their team checks it before approving the return.
According to the English page, you message +966920000244 on WhatsApp or send an email quoting your order number, while on the Arabic page there's a link to a returns portal for submitting the request, after which the courier gets in touch. The courier must receive the parcel from you within 48 hours, and keep a photo you take of the item at the moment of handover. Altered items, for example a watch whose strap was shortened, are excluded from both returns and exchanges.
Exchanges are handled as returns: send the item back, receive your refund, then place a new order for whatever you'd like instead.
Source: Cardial: return and exchange policy