install a new modem

For Preferred Tier and up you're not likely going to get good performance with a SB5120.  I would look into one of the devices on this list if you're looking to swap modems:  http://ww2.cox.com/residential/support/internet/article.cox?articleId=b2ec95d0-7ef9-11df-5590-000000000000.  Usually you can just connnect the new modem and answer a few questions about your account when you try to go online to swap the modem or speak to our technical support team and provide the new information.