We have a support article that explains how to compile the firmware:
https://opensprinkler.freshdesk.com/solution/articles/5000165132-how-to-compile-opensprinkler-firmware
You can launch Arduino IDE and click on compile as Shawn said. More recently I’ve been using Arduino Make (arduino-mk) to compile in command line, which is faster as you don’t have to launch Arduino IDE any more (and I use an external editor anyways since Arduino IDE is not really my thing…)