I needed to upgrade a Cisco WLC 2504 to AireOS version 8.5 and there's a note on the Cisco.com download site that a Field Upgrade Software (FUS) version 1.9 or higher is needed.
Cisco 2500 Series Wireless Controllers Release 8.5 Software.In order to use 8.4 or higher code, you must upgrade the 2504 Wireless Lan Controllers to FUS version 1.9 or higher, this must be done before installing the new AireOS version.
There's a separate link or download area for the WLC FUS firmware.You can't find the FUS version in the WLC web GUI and the only way to get this info is via the CLI.
You use the WLC show sysinfo command to get the FUS version info. Look under the Bootloader Version (1.0.20) and Firmware Version (PIC 20.0). Since I'm running the latest FUS version on the WLC (as of this writing), I don't need to perform an FUS upgrade.
Refer to Table 2 on this link for the FUS release notes. This link provides the steps in performing an FUS upgrade.
(Cisco Controller) >show sysinfo
Manufacturer's Name.............................. Cisco Systems Inc.
Product Name..................................... Cisco Controller
Product Version.................................. 8.2.130.0
Bootloader Version............................... 1.0.20
Field Recovery Image Version..................... 7.6.101.1
Firmware Version................................. PIC 20.0
Build Type....................................... DATA + WPS
System Name...................................... WLC01
System Location..................................
System Contact...................................
System ObjectID.................................. 1.3.6.1.4.1.9.1.1279
IP Address....................................... 192.168.7.10
IPv6 Address..................................... ::
Last Reset....................................... Software reset
System Up Time................................... 0 days 1 hrs 41 mins 35 secs
System Timezone Location.........................
System Stats Realtime Interval................... 5
System Stats Normal Interval..................... 180
Configured Country............................... SG - Singapore
Operating Environment............................ Commercial (0 to 40 C)
Internal Temp Alarm Limits....................... 0 to 65 C
Internal Temperature............................. +47 C
External Temperature............................. +50 C
Fan Status....................................... 3800 rpm
State of 802.11b Network......................... Enabled
State of 802.11a Network......................... Enabled
Number of WLANs.................................. 0
Number of Active Clients......................... 0
Burned-in MAC Address............................ DC:EB:94:95:12:34
Maximum number of APs supported.................. 75
System Nas-Id.................................... WLC01
WLC MIC Certificate Types........................ SHA1/SHA2