Last Name Change on Ticket

Does anyone know how to change a last name on my southwest ticket? My father in law booked a family vacation and used his rewards account, but put my husband’s last name as my last name. I never changed my last name, and still have my maiden name. When I called they said I had to do it online, but it’s all under his rapids rewards account and I can’t find how to just change my last name on my ticket.