Erase Broken Phone

My screen is cracked and I can't enter my passcode because only half the screen works. I received a replacement phone but now I need to erase/reset my old phone so I can send it back but, I don't know how to do that since the screen is broke and I can't gain access. I tried erasing from my computer but the phone needs to be connected to WiFi, which I can't do either. I have to send the broken phone back so I am not charged the fee. Help! How do I get this phone reset??