Após entrar no terminal como root
#nano /etc/apt/sources.list
Copiar o conteúdo entre as linhas
Em preto utilizando apenas softwares 100% livres.
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Types: deb
URIs: http://deb.debian.org/debian/
Suites: testing testing-updates
Components: main contrib non-free non-free-firmware
Signed-By: /usr/share/keyrings/debian-archive-keyring.gpg
Types: deb
URIs: http://security.debian.org/debian-security/
Suites: testing-security
Components: main contrib non-free non-free-firmware
Signed-By: /usr/share/keyrings/debian-archive-keyring.gpg
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