During the 30 days you can only exchange the phone at the original point of purchase. For Best Buy, that means you can go to ANY BB store and they will exchange it; however, for Sprint, you have to take it to the exact Sprint store or kiosk you purchased it.
After the 30 days no where will exchange it unless you have insurance. If you have Sprint insurance, they send it off to be repaired or you get a refurb. If you have Best Buy insurance, they will send it off for repair or give you a new one. Hope that helps.