The github page has links to how to compile:
https://github.com/OpenSprinkler/OpenSprinkler-Firmware
If you use OSPi, the instructions for installing firmware is here:
https://openthings.freshdesk.com/support/solutions/articles/5000631599-installing-and-updating-the-unified-firmware-on-ospi
as to how to get pull request, just google.