get apt cert
wget https://www.virtualbox.org/download/oracle_vbox_2016.asc
-----BEGIN PGP PUBLIC KEY BLOCK-----
Version: GnuPG v1.4.12 (GNU/Linux)
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-----END PGP PUBLIC KEY BLOCK-----
install cert
apt-key add oracle_vbox_2016.asc
add apt source list
nano /etc/apt/sources.list.d/virtualbox.list
deb http://download.virtualbox.org/virtualbox/debian stretch contrib
install
apt update apt install virtualbox-5.1