Just came across a really cunning trick to update the firmware for an HP OfficeJet 6500A Plus E710n printer / scanner / copier / fax device. Its the All-in-One Plus model, aka product code CN557A.
On the status page, the device was reporting firmware version CIP1FN1126AR - essentially release 1126. Checking HP downloads shows a newer firmware, release 1206. Note that the firmware won't show on the support website if you specify your OS as Windows 10 - choose Win 7 instead.
When you download and run the latest firmware updater from HP, a file called OJ6500-E710n-z_R1206A.exe it spots the printer correctly on the network or WiFi. However the update button is greyed out when you specify the printer you want updated. No matter what compatibility settings / firewall / OS / computer / ethernet / WiFi / USB combination you try it won't let you update.
Here's the cunning bit - go into Control Panel > Printers, find the 6500A to update, right click and make sure its shared, preferably with a short name and no spaces, eg 6500A
Now use Winzip or 7-Zip to open the OJ6500-E710n-z_R1206A.exe file, locate the .FUL file and extract/copy it to the desktop. I'd right-click and rename it to something much shorter, eg 6500A.FUL
Then start a Command Prompt - easiest way is to go to Start > Search (or Run) and type
In the black DOS box that comes up, type a couple of commands ...
copy /b 6500A.FUL \\PC\6500A
[note that PC is the name of your computer, you can find it in Control Panel > System]
This sends the file as a binary transfer direct to the printer and forces the update. You should see the printer go into an update mode and restart. Takes a few minutes for it to complete