The simple upgrade involves replacing one or both of the small standard antennas that come with the router, with a larger, high-gain antenna:
Cisco Linksys WiFi access points for home and small office, have two RP-TNC antenna jacks in the rear of the router, which come with two 5dBi omnidirectional antennas attached to the connectors. It is easy to remove those antennas, or one of them, and thereby connect a much larger antenna which you can position higher or outside the house or building, thereby gaining much better line-of-sight to the end-users who will connect to the wireless access point.
RP-TNC is not one of the most common WiFi antenna connector types and so you will not find a lot of antenna options that specifically have an RP-TNC: Most WiFi antenna options have an RP-SMA connector.
How to connect an upgraded antenna: Connect the female connector of an RP-TNC extension cable to one of the antenna jacks at the rear of the WAP, and then connect the male end of the RP-TNC cable to the new, upgraded antenna: You will experience a huge increase in signal strength for both the TX (transmit) and RX receive, from this simple upgrade and re-positioning of the antenna.