The normal Racket distribution includes ".rkt" sources for collection-based libraries. After modifying library files, run raco setup (see raco setup: Installation Management) to rebuild installed libraries.
The normal Racket distribution does not include the C sources for Racket’s run-time system. To build Racket from scratch, download a source distribution from http://download.racket-lang.org. Detailed build instructions are in the "README.txt" file in the top-level "src" directory. You can also get the latest sources from the git repository at https://github.com/racket/racket, but beware that the repository is one step away from a normal source distribution, and it provides build modes that are more suitable for developing Racket itself; see "build.md" in the git repository for more information.