Call the Apple Store and ask. That's the only way you're going to know for sure. Most likely if the order hasn't been sent for processing and shipment preparation you can still make alterations, but only Apple can tell you that with certainty. If they've already charged your credit card, then the order has most likely already been sent to shipping in which case it's probably too late to make alterations and you'll have to return the unit if you don't want it and pay the restocking fee.
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Items in your order will be divided into items that are eligible for cancellation online and items you cannot cancel. For each item that you can cancel online, you will need to select the item and a reason for cancellation. Once you’ve submitted your cancellation, we’ll send an email with your cancellation details.You can also view your cancellation on the order details page of the order containing your cancelled item(s) – the status of the items will read “Cancelled”. You will not be billed for these items."
It's not clear if they give me my money back or they save this money for a future order!
In this moment I'm abroad and I can't call!
If you don't see the option to cancel, the item has already shipped or it's being prepped for shipping. If you can still cancel, they will reverse the charge on your credit card.
My personal understanding is they don't actually charge you until shortly before it ships. They may place a preauthorization for the funds with the credit card company. If you haven't been charged yet, and only have a pre-authorization, that hold will be released.
If they have charged you, and you cancel the order, the funds will get refunded. In some situations such as you already received it, and return it opened, there may be a restocking fee.