Can you put the router on the 2nd floor? I assume not if you have FIOS as the main line is probably only routed to 1 location.
Are you using a router/modem combo? If so, I would look at getting a dedicated router. In my experience, every combo unit I have ever had has been piss poor in terms of WiFi. If you have a dedicated router, how old is it? Does it have dual or tri-band? If so, set one channel to 2.4GHz and another to 5GHz. 2.4GHz (based on my understanding) is better for long range but has slower speeds. 5GHz sacrifices range in exchange for higher speeds.
I would also look at how many networks are nearby and try to select a lesser-used channel. I can probably see 12 networks in my area (not sure how as it's a single-family home) but I have adjusted the WiFi channel to really only overlap with 1 or 2. I can't remember what program I was using for that, but they're out there. Some are free on the app stores for mobile.
Also, some routers can have their transmission level upped if you login to the UI of the router.