Let me guess....
You are currently using the Gopro App on your smartphone with wifi? I assume you are using a FC40 on 5.8GHz since we're at the FC40 forum here.
I wouldn't really bother to increase the wifi range, the gopro wifi has way too much lag to use as FPV solution. But if you want, do a search here or on youtube. There are lots of folks using wifi repeaters to extend the range considerably, although everybody I know here uses the FC40 cam on wifi, unless they have a regular FPV Tx/Rx system.
A decent FPV setup will set you back for at least 250 bucks [edit: make that 300] for a video transmitter 2,4GHz, 1.2Ghz or 900Mhz, a receiver, a screen, a screen mount and battery. You can take the FPV image from the gopro on your screen and reach distances over 2 km with the proper antennae.