How do I find out what kind of AirPort Extreme I have?

I had a 2T HD connected to my AirPort Extreme to use as a Time Machine backup - The 2T HD was too small for my home network, so I upgraded to a 6T HD. Now I can't connect to my 6T Time Machine drive to back up. I read a Support link that said the time machine back up only works with a AirPort Extreme 802.11ac model only. How do I find out what kind of AirPort Extreme I have. If my AirPort Extreme is "out dated" will upgrading to the 802.11ac allow me to use my 6T external HD?