Hi,
You are correct that you can’t upgrade OS 2.1 to firmware 2.1.6 through USB any more. This is explained in this support article:
https://opensprinkler.freshdesk.com/solution/articles/5000668400-updating-opensprinkler-2-1-to-firmware-2-1-6-and-beyond-
We do provide firmware upgrade service free of charge. Alternatively, we provide discounted price for OS 2.1 owners to upgrade to OS 2.3.