I am not sure exactly what you are asking. The IP of your router (if you bought one) is factory set. If you want to change it look here (http://www.dslreports.com/faq/6742).
Easier than trying to write it all out. If what you are asking is how to change the IP assigned to your modem by your ISP, you usually can not. However the IP for the modem is usually a dynamic one and is updated and reassigned by the ISP periodically. As you can see from the article if you could randomly assign an IP to the modem, your ISP would loose contact with it. So the modem IP needs to be controlled by the ISP/servers it communicates to. Hope this helps.