When you use the web interface for Airdroid by signing in at web.airdroid.com, unless you scan the QR code with the device you want to connect to, Airdroid will connect to your default device.
If you want to change the default device that Airdroid connects to, it's easy. Just log in to the web interface using one of your Airdroid devices and click the gear icon on the top bar.
Next, click on the "Devices" tab.
Finally, just select the device you want to make your default Airdroid device from the drop down list.