Linette,
This can get confusing, since we are talking about two different things here.
It is possible to turn off the wireless function on the FIOS router, connect the AirPort Express to the FIOS router using a permanent Ethernet cable connection, and use the Express to provide your wireless network.
That's a lot of work, with no assurance that there would be any improvement if you did this. Unless you are confident in your abilities and adept at troubleshooting, it would probably be best to leave well enough alone and use the FIOS router for your wireless service.
It appears that Allen was suggesting that you use the current FIOS router to provide a wireless signal and the Express (located a few rooms sway) to "extend" the FIOS router signal wirelessly.
That won't work, since the Express is not compatible with the FIOS router for this purpose.