The first step was locating the firmware. A helpful blog post I found that identified the latest firmware as being a download file named
apc_hw02_aos394_rpdu_393.exe. Knowing that the firmware downloads begin
XXX is the version number), with a bit of Google trial-and-error incrementing the version number, leads to the APC website and confirms that number seems to be the latest firmware.
Once I had the firmware downloaded, for ease I just updated using a Windows laptop - running the downloaded file prompted to extract it then automatically ran the updater. The updater asked for the IP, username and password of the PDU then did the update an exited. I simply repeated this process for all 3 PDUs.