Airport extreme does not show up on Airport utility
I have recently bought an Airport Extreme from Ebay model No. A1143 (i know old generation). I currently own a late 2013 iMac running OS X 10.10.4 Yosemite running Airport utility version 6.3.5. When I connect the AE I've noticed that it does not show up under the globe in airport utility..however it is recognize under other wi-fi devices...Once I click on the AE it goes through it configuration and pairing and then it tells me that "This version of Airport Utility requires this Airport Extreme use firmware version 7.3.1 or later" how can update this firmware if i cannot get access to the AE via airport utility? Help.
iMac, OS X Yosemite (10.10.3)