There are 2 main places for source code and application.
If you want the application, go here: https://github.com/farmerbriantee/AgOpenGPS/releases and download the application. Unzip it and run AgOpenGPS.exe
If you want the source code you have 2 ways. Go to the main github page https://github.com/farmerbriantee/AgOpenGPS Click on clone or download and select download zip. Unzip the file and run AgOpenGPS.sln to open in Visual Studio.
You can also click on the source code on the release page to download the source code associated with that particular release.