Actually, the law works in your favor here, so it's really not a question of whether they'll let you, but how to do it properly! We have 14 days to return goods of satisfactory quality if they don't suit us - this doesn't require any proof of defects. The main condition is that the item shouldn't be used, has no damage, and you need to have a receipt or warranty card.
In your case everything checks out - a week has passed, the box is intact, the phone is in perfect condition! Just go to M.Video with the phone, box, all accessories (headphones, cable, if there were any) and your receipt. Say you've decided to return it because the model doesn't work for you - that's enough. Sometimes they ask for a reason, but they can't demand a detailed explanation or refuse just because you "didn't like it." If they try to give you a hard time, calmly remind them about consumer protection laws!
I had a similar situation with a tablet - the screen turned out to be too small for me, and I used it for literally two days. The salespeople initially wanted to make a fuss, but when I mentioned my right to return it, everything went smoothly in twenty minutes! The refund usually goes to your card or in cash - clarify whichever works better for you.