[ @main @update ]
Сперва убедись, что нужный тебе пакет есть в https://packages.debian.org/stretch-backports/
Add backports to your sources.list
deb http://ftp.debian.org/debian stretch-backports main
or add a new file with the ".list" extension to /etc/apt/sources.list.d/
You can also find a list of other mirrors at https://www.debian.org/mirror/list
Run apt update
Install a package from backports
All backports are deactivated by default (i.e. the packages are pinned to 100 by using ButAutomaticUpgrades: yes in the Release files. If you want to install something from backports run:
aptitude -t jessie-backports install "Your_package"
sloppy distributions
To guarantee a clean upgrade path from a previously stable distribution to the next stable it is not allowed to upload packages from the current testing to that backport distribution. To allow some newer packages for those systems we create so called sloppy distributions. For oldstable (wheezy) packages from the current testing (stretch) are allowed to be uploaded to wheezy-backports-sloppy.
You can use it exactly as the current backports distribution, just add another line to your sources.list and install new packages via apt -t wheezy-backports-sloppy install or apt -t jessie-backports install
Если не работает apt update…