If you have the source code, then you can open those files in any text editor. Source code files are just plain text files. There are, of course, different editors and IDEs (Integrated Development Environment) that are taylored to specific languages, but all they do is open the source files (as text files) and highlight keywords and stuff with special colors or fonts to help you read it. But you don't need that to open source files, any text editor (e.g., Notepad) will do just fine.
If you don't have the source code of the application, then you cannot open it. I mean, applications don't contain their own source code. The source code for a specific open-source application is usually downloadable as a separate package (usually a zip file or tar-ball, or hosted on a version control server with tools like svn or git). You need to get the source code first, and then open it in a text editor or something better.
If it's not an open-source application, then you cannot get the source code at all, that's the whole meaning of "closed-source" applications. You can run a dissambler program on the executables, but that's only gonna translate machine code into assembly listings, which isn't really useful unless you know assembly already, which I doubt, since you wouldn't be asking this question if you knew assembly at all.