Replace internal PCB antenna with external antenna
For this specific design, you have 2 antennas. 
As replacement, you have 3 options, in order of reliability:


Find whatever SMT connector fits the 2 empty footprints and a suitable adapter to your antenna and adjust the resistors accordingly.
Solder a piece of wire, ~1/2 Wavelength to the relevant pad.
Cut the antenna connector and solder the coax directly to the pads.


Don't expect miracles. This is an unreliable connection at best, and might destroy your board at worst.