How to modify debian packages

https://askubuntu.com/questions/28372/how-do-i-get-and-modify-the-source-code-of-packages-installed-through-apt-get

Use the command apt-get source <package> (don't use sudo with it) to download the source of a package.

From man apt-get:

   source
       source causes apt-get to fetch source packages. APT will examine the
       available packages to decide which source package to fetch. It will then
       find and download into the current directory the newest available version of
       that source package while respect the default release, set with the option
       APT::Default-Release, the -t option or per package with the pkg/release
       syntax, if possible.

       Source packages are tracked separately from binary packages via deb-src type
       lines in the sources.list(5) file. This means that you will need to add such
       a line for each repository you want to get sources from. If you don't do
       this you will properly get another (newer, older or none) source version
       than the one you have installed or could install.

       If the --compile option is specified then the package will be compiled to a
       binary .deb using dpkg-buildpackage, if --download-only is specified then
       the source package will not be unpacked.

       A specific source version can be retrieved by postfixing the source name
       with an equals and then the version to fetch, similar to the mechanism used
       for the package files. This enables exact matching of the source package
       name and version, implicitly enabling the APT::Get::Only-Source option.

       Note that source packages are not tracked like binary packages, they exist
       only in the current directory and are similar to downloading source tar
       balls.
To build a package from source, first install the build dependencies:

sudo apt-get build-dep <package>
Then use dpkg-buildpackage to create a .deb file. From APT and Dpkg Quick Reference Sheet:

dpkg-buildpackage Builds a Debian package from a Debian source tree. You must be in the main directory of the source tree for this to work. Sample usage:

 dpkg-buildpackage -rfakeroot -uc -b
Where -rfakeroot instructs it to use the fakeroot program to simulate root privileges (for ownership purposes), -uc stands for "Don't cryptographically sign the changelog", and -b stands for "Build the binary package only"

In a terminal, cd into the directory containing the package source (e.g ~/code/hellanzb-0.13) and run the following command:

dpkg-buildpackage -rfakeroot -uc -b
If the build is successful, there will be a .deb file located in the parent
directory (e.g ~/code/hellanzb_0.13-6.1_all.deb).