If it has shipped then to cancel it, you'll need to return it for a refund.
BUT, you can try this within your 21 days:
If you ordered your system online, then Dell's Sales and Customer Service will tell you that they can't offer you the online prices. OK, fine - but it doesn't mean they can't fix your order. Dell Sales can modify an online order without losing the discounts and coupons - I know, on 1 of my orders (that I subsequently cancelled)
I called to add the 1450 wireless card to the configuration.
Don't threaten. If there's a better price for the exact same configuration or if you want to update the warranty then call and ask. When they say no then you politely ask: "Do I really need to return the system just to take advantage of the better price or update the warranty? I have no problem with returning the system just to re-order it again. The only one who will profit is DHL from all the shipping. Dell doesn't want my system back. What can we do?"
That's basically what I did on my system (below)
. You can reread the details of my recent experience in this thread
. It can be done. I was calm and polite, and I did all the math prior to calling. I had all the details and math printed out to help me prove my case when I called. I was successful in getting a credit on my credit card for the difference in price.
The important thing is to do this between ordering the system and before
the 21 day satisfaction period expires. I really recommend not to threaten - just be insistant.