To upgrade the firmware on your Iaxy, simply update your Asterisk code from Digium's SVN server. This will download the newest Iaxy firmware. Compile the Asterisk code by doing a "make clean install" and then restart Asterisk if it is currently running after the compile is finished. When the Iaxy registers to Asterisk after the restart it will notice there is a new firmware and it will automatically install the new firmware. You should see the orange LED on the Iaxy lit up when it registers and it will turn off for a few seconds and come back on after the upgrade of the firmware is finished. To check what version of the firmware Asterisk is giving out to Iaxy's you can do "iax2 show firmware". You can also check to see what version of the firmware an Iaxy is running by looking at "iax2 debug". It will look something like so:

Tx-Frame Retry[000] -- OSeqno: 001 ISeqno: 002 Type: IAX Subclass: REGACK
Timestamp: 00027ms SCall: 00001 DCall: 05741 [216.207.132.168:4569]
USERNAME : test
DATE TIME : 160388975
REFRESH : 60
APPARENT ADDRES : IPV4 216.207.132.168:4569
FIRMWARE VER : 22