sed -Ei

  sed -Ei 's/^# deb-src /deb-src /' /etc/apt/sources.list

See also

• apt update
• sed, gsed, -n, -i, Ansible: replace, append, Chef: Editing a file‎‎, lineinfile
• /etc/apt/, /etc/apt/sources.list.d/, deb, sources.list