@aruzsi: for OpenSprinkler 2.1, the instructions are:
1) Unplug 24VAC, unplug USB (if it’s already connected)
2) Plug one end of the USB cable to your computer
3) Press and hold button B2, plug the other end of USB to OpenSprinkler
4) Release B2 two seconds after USB is plugged in
The LCD screen should remain off. If it lights up, that means you have to hold B2 a bit longer. The device should show up as a USBasp device, and if you are in Windows, you will be promoted to install driver if driver is not already installed.
For additional details, please check the firmware update tutorial: