As Edg1 said, contact Verizon and they will help you. I suggest you try chat.
Note that if you have FiOS ONE set-top-boxes, additional boxes can be connected by WiFi, coax or Ethernet. This can be very handy if you don't have coax in a room.
They will ship you an additional FiOS ONE box without requiring a tech visit. If you don't have FiOS ONE and want to upgrade, they will ship you a set of boxes; again without a tech visit. There is an upgrade fee and there may be a change in rental rates and/or other service charges.
FiOS ONE requires a G1100 or G3100 router. If you don't have one, you'll need to upgrade your router, too.
You can also do this through their online ordering system, but chatting with them may get you a better deal.