The phone should already be carrier-unlocked, but check with Verizon first. You should then be able to insert an AT&T SIM and start using it. But be aware that LTE reception might be very spotty, since the phone only supports bands 4 and 13, while AT&T uses 2, 4, 5, and 17. If band 4 is strong in your area, it should be fine, but you might have issues elsewhere.