I am assuming you are speaking about a custom ringer, if this is the case then you should be able to open the contacts information and select edit from menu and change the custom ringer option.
Do you mean purchasing ringtones and they went to the wrong phone?
If you purchased online via MyVerizon, you need to register each line on the account for a separate MyVerizon account. If you don't, all the purchases end up on the primary line.
You can call Customer service and have them apply the ringtone to the correct line - (800-922-0204) or *611 from your handset.