The iLO web interface allows you to update the iLO firmware simply uploading a local .bin image file,
unlucky the HP support website it's a real mess and barely usable..

So in order to obtain the image file first you will have to download some sort of self-extracting updater packaged for your operating system (yes, no jokes) anyway thanks to google i was able to find some official HP releases:

ILO4:

• ilo4 2.10
• ilo4 2.03
• ilo4 2.02
• ilo4 2.00
• ilo4 1.53

ILO3:

• ilo3 1.82
• ilo3 1.80
• ilo3 1.70
• ilo3 1.65
• ilo3 1.57

Then you will have to unpack the contents of this evil and undocumented self-extractor (without executing the useless installer) using the --unpack flag.

For example to update an iLO4 machine:

~ $ wget http://ftp.hp.com/pub/softlib2/software1/sc-linux-fw-ilo/p1457726527/v101197/CP024540.scexe
~ $ sh ./CP024540.scexe --unpack=/tmp/ilo
CP024540.vmcfg
CP024540.xml
ESXi_5.5/
ESXi_5.5/flash.so
ESXi_5.5/hpsetup
flash.so
hpsetup
ilo4_210.bin
README.TXT
OS Version found  [3.18.6-1-ARCH]

Finally the firmware image ilo4_210.bin it's here, suitable for the upload on the iLO web interface.

We are done here... thanks HP!