Of u buy a phone in cash and u still exchange it for a different one?

Yes, your can

If I remember best buy policy if you buy a unlocked phone you can return with in 15 days for full refund. If you buy Verizon or Sprint phone you have up to 1 month

That depends on who it was purchased with sometimes you can get credit for one for to another depends on condition and of the phone and weather or not the company want to give a credits or not

How about if it's been 1 month ?