The actual distribution of ngspice may be downloaded from the ngspice download web page. The installation for Linux or MS Windows is described in the file INSTALL to be found in the top level directory. You may also have a look at Chapt. 32 of this manual for compiling instructions.

If you want to check out the source code that is actually under development, you may have a look at the ngspice source code repository, which is stored using the Git Source Code Management (SCM) tool. The Git repository may be browsed on the Git web page, also useful for downloading individual files. You may however download (or clone) the complete repository including all source code trees from the console window (Linux, CYGWIN or MSYS/MINGW) by issuing the command (in a single line)

git clone git://git.code.sf.net/p/ngspice/ngspice

You need to have Git installed, which is available for all three OSs. The whole source tree is then available in <current directory>/ngspice. Compilation and local installation is again described in INSTALL (or Chapt. 32). If you later want to update your files and download the recent changes from sourceforge into your local repository, cd into the ngspice directory and just type

git pull

git pull will deny to overwrite modified files in your working directory. To drop your local changes first, you can run

git reset --hard

To learn more about git, which can be both powerful and difficult to master, please consult http://git-scm.com/, especially: http://git-scm.com/documentation, which has pointers to documentation and tutorials.