There is no Airport Extreme anymore. Apple got out of that business a few years back and stopped all support for existing hardware, so no new updates or anything. If your router is working fine, then stick with it.
If you need an upgrade to the newer technologies (802.11N, dual-band, multi-antenna, MIMO, etc.) then you'll have to switch brands to a different manufacturer.
I recently upgraded to a TP-Link Archer 20 from Airport Extreme and it's working quite well.