If you no longer have the old device, try the Duo Restore process on the new device. Additionally, if you are getting a new phone, if you add your new phone to Duo before wiping and trading in your old phone, you will have a smoother transition experience.
You can add multiple devices to Duo, such as an iPad as a backup.
To add a device for the first time, visit: Duo 2FA: Mobile Phone Setup. After you have set up Duo for the first time, you can add, remove, or change your devices.