You can install or upgrade a binary package with the apt-get install package-name and apt-get upgrade package-name
commands.
You can obtain the current version of a source package with the
apt-get source package-name command. This unpacks the
source package, but does not attempt to compile it.
You can obtain, and compile, the current version of a source package
with the apt-get source --compile package-name command.
For this to do something useful, you will want to specify meaningful
options in the /etc/apt/apt.conf files.
For instance, the following option tells the system to compile
and build the binary package(s), but not to sign the changelog.
(The -rfakeroot option allows me to compile packages as an
unprivileged user.)
/etc/apt/apt.conf
DPkg::Build-Options "-rfakeroot -b -uc";
This option tells the system to compile and build the binary package(s), and to sign them with my GPG/PGP key. This requires me to provide my passphrase for each source package downloaded and compiled.
/etc/apt/apt.conf
DPkg::Build-Options "-rfakeroot -b -m'Bear Giles <bgiles@coyotesong.com>'";
Finally, a "compilation server" can set up to compile and sign packages with a passphrase-less GPG/PGP key.
/etc/apt/apt.conf
DPkg::Build-Options "-rfakeroot -b -m'Compiler <cserver@coyotesong.com>'";