How can I get my Macbook Air 2013 (September 2013) to connect to wi-fi?
Something is wrong here!

It is correct there are always software updates, but it is not possible that your Airport (WiFi) does not work at all (unless it is broken) and no software will fix that, so you need to send your computer back.

Make sure it is turned on in your system preference - Network- WiFi ON!

Now to the software updates:

Even if you bought it tomorrow does not means it has the latest software updates.

Since you do not have a WiFi Internet connection you can not get the software updates via Internet.

You could bring it back to Apple and they will fix it.

Alternatively, get a piece of Ethernet cable and connect to the router.

Since it is a MBA that does not has a Ethernet plug, you will need a Thunderbolt to Ethernet plug.

Then Click on the Apple (top left) and on the software update.