Upgrade my modem I got an email stating that it's outdated that you guys are willing to send me a new one for free

Ain't got nothing else stating that my modem or my Internet modem is outdated you guys are willing to send me a new one for free I want to go and do that but I also do not want to be charged for the old motor so I need to know the steps on how to do this so I'm not charged extra or my bill doesn't go up at all because of it oh if I could just be told the steps as to how to go about it where I send it so my bill stays the same and I get a new upgraded modem I will be willing to do that