If you want to download ringback's directly from the handset you'll need to download the VZW Tones Deluxe Application. You can typically find this application by going through the main menu in the phone to get to Media Center > Music & Tones > Get New Ringtones > Get new applications.
Keep in mind if you're on a Nationwide plan and you don't have a data plan you will be charged megabytes for downloading this application as well as browsing and downloading tones.
If you want to avoid the megabyte charge the other option you have is to purchase and manage your RBT's through MyVerizon. You'll need to register each number for their own MyVerizon account. These additional lines that you register online will not be able to make any major account changes or see any account detail, they'll only be able to manage their own individual phone number.
1. How do I purchase a Ringback Tone? Ringback Tones may be purchased through the Internet, and through text messages to the Ringback Tone shortcode (728), use "RBT", "LIST", or the 3-digit song code in the message.*