You can call B&R directly and talk to their tech support. It’s free. Their headquarters is in Georgia and they have many other branches throughout the USA.
The PLC will most likely not have the source code on it. Like jaden said, it is compiled then downloaded. Best you can do is try to contact the OEM for a copy of the program or at the very least a printed version.
The hardware manual is available online with googles help.
Manual states you need B&R PROGramming SYStem software, but good luck finding it. Best yet, good luck finding a PC running MS-DOS 2.11 or better.