I think to upgrade these you need actual ROMs though, as opposed to just being able to find the OS online somewhere - I have RiscOS 4.39 ROM images for use with the Virtual Acorn emulator but as far as I know there's no way to flash them to the existing ROMs. Buying RiscOS 4 ROMs is quite an expensive business, not really worth it for a free machine Do you know which model of RiscPC it is? They are cool machines; I found a RiscPC 700 with a PC card and a load of manuals/discs on Freecycle a while back.
You can get them direct from RiscOS Ltd but they're extremely expensive. I saw a set on eBay once which is what I was thinking of when I said they're quite expensive, as I think they got bid quite high.