The short answer is: it depends what OS you're on, and where your Django package is installed.
On Unix-like systems, it'll be somewhere like lib/python2.7/site-packages/django/contrib/ (substitute your python version, obviously). But what goes before 'lib' depends on where your Python libraries in general are installed, which would vary from OS to OS; if you're using virtualenv, it'll be the root directory for the environment you're working on, etc.
You shouldn't need to change anything in there, however. What leads you to look for it? If you just want to browse the code, then I'd recommend looking through it at github:
https://github.com/django/django/tree/master/django or else cloning the relevant branch from there.