Please visit the following google drive location to download the operating system image for your respective single board computer.
Download the image along with the latest software for your Single board computer.
Use win32 disk image (or any other image writer application) to write “.img” file into the SD card. Please back-up data from the SD card as it will be completely formatted after this operation.
Once the Image is written remove the SD Card from your computer and insert it into your single board computer (SBC).
Please refer below document for more information