"W: There is no public key available for the following key IDs:9D6D8F6BC857C906"

Try Debian 7 Wheezy (EOL) repositories:

deb http://ftp.us.debian.org/debian/ wheezy main contrib non-free
deb-src http://ftp.us.debian.org/debian/ wheezy main contrib non-free

deb http://ftp.us.debian.org/debian/ wheezy-updates main contrib non-free
deb-src http://ftp.us.debian.org/debian/ wheezy-updates main contrib non-free

deb http://security.debian.org/ wheezy/updates main contrib non-free
deb-src http://security.debian.org/ wheezy/updates main contrib non-free
(generated by https://debgen.xyz/)

it belong to /etc/apt/sources.list
comment out or remove other lines

then:
apt-get update
apt-get upgrade