How can I increase signal from airport

The AirPort is configured at the factory to produce the maximum amount of power permitted by law. You can only reduce it, if it's too much.

If your AirPort network is insufficiently large for your installation then you can add more AirPort Base Stations to extend your wireless network. Read:

Wi-Fi base stations: Extending the range of your wireless network by adding additional Wi-Fi base stations

Wi-Fi base stations: Setting up and configuring a roaming network (802.11 a/b/g/n)