Changes are easiest before an order ships, so act quickly.
We do not process orders, so requests go to the official seller using the contact details in your confirmation email. State your order number and the change you want, and ask for written confirmation.
If the order has already shipped, ask about redirecting it with the carrier.
Keep screenshots of your order and your messages, with dates. The guarantee runs from purchase, so note your order date. If you cannot reach the seller, your card issuer has a dispute process, which is a last resort.