To be fair, I do not like to provide support for OpenSprinkler clones (especially TIAO, since they don’t even want to acknowledge us on their product page). However, your question is an easy one to answer. Firmware instructions are on the OpenSprinkler.com website –> Support page. For OSPi, it’s on this specific page:
https://openthings.freshdesk.com/support/solutions/articles/5000631599-installing-and-updating-the-unified-firmware