Did your ISP provide the router or is it your own? If the ISP provided it, contact them for assistance with the firmware update. They may have specific firmware that is needed so the device remains compatible with their services.
If this is separate from your modem and is a router you own and installed, then reach out to the manufacturer Netgear. They can assist you with the getting the update applied.
Keep in mind that upgrading your bandwidth or updating the firmware is not going to have much, if any, affect on the WiFi signal in the back of the house. This is from signal loss due to distance and/or obstacles (walls, interference, etc).
Instead, you would want to look into setting up a mesh network or adding WiFi extenders to bring a stronger signal to these areas.
Hope that helps!