Hey Charles,
the image files for SD cards are here. The image files are the .img.xz files.
You need a PC with a card reader/writer and a SD card. You download the .img.xz file of the version you want to the PC. On a unixoid operating system, you unpack the file with ‘unxz’ and write it to the SD card device with ‘dd’. On other operating systems you do it like this:
Then you insert the SD card into its slot on the underside of the Raspberry Pi 3B inside the Onefinity Controller:
WARNING: Make sure that your Onefinity Controller is disconnected from mains power before opening it. Simply switching it off does not interrupt the power to the internal power supply!
Welcome to the forum!