You will need to populate the U.FL connector at J11. This is Molex part number 0734120110 available from Digikey.
next you need to disconnect the onboard antenna and connect the U.FL connector to the circuit. Here you have two options on how to do it. 1) for a non-powered antenna, remove R126 (zero ohms) and short the pads at C217. 2) for a powered antenna you will need to install a 22pF capacitor at C216.
R126 and C216 share a common pad and are placed at right angles to each other so only one of the two can be populated at any given time.
The inductor (L19) should be 100nH and the capacitor (C217) should be 1uF. C217 is a filter capacitor. These two components provide power to your active GPS antenna. If you are not using an active antenna (no internal amplifier) then you don't need these components installed. Not all powered GPS antennas will run from 1.8V make sure you select one that does. Also check that the SW turns on regulator L4 at 1.8V to power an active antenna.
It should be noted that I haven't tried this out so I am not sure it will work!. Good Luck.
Full Disclosure: I am an employee of Qualcomm Canada and any opinions I may have left in any post do not necessarily match the opinions of my employer.