Can I use a recently bought sprint iPhone 6 plus for att?

I have bought it and I have then I phone 6 for att and my girl friend bought the iPhone 6 plus for sprint can I just switch SIM cards or what do I do?

Nope. Those phones are way too new which means they are still locked to the specific carrier. Swapping SIMs will do nothing. ATT uses GSM signals, Sprint uses CDMA. Unless each phone is unlocked, which won't be the case, it can't work. Best luck.