@rhldr:
1. go to https://github.com/rayshobby/opensprinkler, and you should see a ‘Download as Zip’ button on the right-hand side.
2. firmware update instructions are on the OpenSprinkler homepage: http://www.opensprinkler.com. Click on 6. Firmware Update link on the left-hand side.
@rwp:
If you just want to flash a pre-compiled firmware, you do not need to install VirtualBox — the VirtualBox approach is meant for users who want to compile or modify the firmware source code. Each firmware is independent and does not rely on previous versions of firmware. As I said above:
firmware update instructions are on the OpenSprinkler homepage: http://www.opensprinkler.com. Click on 6. Firmware Update link on the left-hand side.