If i buy my iMac online BTO and i decide to replace because of some defectiveness, how would the return proccess be? could i take it to my local apple store even though i bought it online? if not, how do i send it back? also what is the return policy if i buy applecare later without the imac? would i still be able to replace it if it comes defective? How long from the date of purchase do i have to do exchanges? thanks!