You can download the distribution zip file from here (last updated 24-05-2015). This zip file contains the program, the empty database engine and all the scripts needed in order to execute it.
Unless otherwise stated here, you have to download the distribution zip file only once. After that you can install the latest version of the program by downloading only the jar (last updated 24-05-2015) of the program and overwriting it on the file with the same name in the installation directory.
In case you want to see or tweak the source code of the program, you can point your browser here and find various snapshots of the code.Follow the announcements and the changelog at freecode or the live code development at github. These files when uncompressed correspond to netbeans project directories so working with them should be easy.