Add the repository non-free-firmware to the APT software package sources in the /etc/apt/sources.list ^[1] file, and do apt update with the root permission.

For example, set the content of the file to this:

deb http://deb.debian.org/debian bookworm main contrib non-free non-free-firmware
deb-src http://deb.debian.org/debian bookworm main contrib non-free non-free-firmware

deb http://deb.debian.org/debian-security bookworm-security main contrib non-free non-free-firmware
deb-src http://deb.debian.org/debian-security bookworm-security main contrib non-free non-free-firmware

deb http://deb.debian.org/debian bookworm-updates main contrib non-free non-free-firmware
deb-src http://deb.debian.org/debian bookworm-updates main contrib non-free non-free-firmware

. And then, install firmware packages that you need, with the apt install command.

Starting from Debian 12 (bookworm), all non- free ^[2] firmware packages, such as firmware-iwlwifi, firmware-amd-graphics, or firmware-misc-nonfree, have been moved from the repository non-free to their own repository, non-free-firmware.

The Debian project has taken the decision ^[3] in October 2022 to create a new repository non-free-firmware and include its content on installation media for Debian 12 ( bookworm ) ^[4] to make things easier for the users. This means that the newer Debian 12 installer will automatically install such firmware along with the operating system if needed.