How to switch from verizon to att with my iphone?

Someone just gave me there iphone 4 and i know i could unlock it but there's no sim card slot so is there a way to change it to att

There's no way to change an AT&T smartphone to Verizon, nor can it be the other way around. Verizon and Sprint phones uses the CDMA network, while T-Mobile and AT&T uses the GSM network.