Most stores will accept a return for any reason within a short period (usually 7 days to a month). However, some will charge you a restocking fee. Usually 10%-15%.
If the other laptop is *exactly* the same, you could simply buy it from the other store, then give it to the first store unopened for a refund. Usually stores will waive the restocking fee if the item is unopened. i.e. You keep the current laptop, but take the laptop you bought at the second store, and "return" it to the first store for a refund. Say you got it as a gift, but it turned out the person already got a laptop without you knowing it, so you're returning it unopened.
You can also try bringing the ad for the lower price ad to the store, and complain. Sometimes they'll match it and refund you the difference in price.
Likewise, if it's the same store which has lowered the price, and they refuse to credit you for the price decrease, you can threaten to do the second paragraph. Say you'll just buy another one at the lower price, and return it for a refund using your first receipt. That'll make them realize the stupidity of not doing the price adjustment for you.
Some credit cards have a price protection feature. If you buy something using the card, and find it advertised at a lower price within some period (usually 90 days), you call the card's customer service department. Send them a copy of the ad with the lower price, and they'll subtract the difference from your credit card bill.