How To Upgrade Debian 8,9,10 to Debian 12 Bookworm
Step 1.) Upgrade to Debian 11 first
The process to go to Debian 12 is not as smooth as 11, when trying to upgrade from Debian 10. In fact, it doesn't work directly, so you'll first need to follow this guide to update to Debian 11, reboot and come back here if successful.
Step 2.) Update sources.list
Update your /etc/apt/sources.list like this:
deb http://........
W: GPG error: http://archive.debian.org squeeze Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY AED4B06F473041FA NO_PUBKEY 64481591B98321F9
W: GPG error: http://archive.debian.org squeeze Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY AED4B06F473041FA NO_PUBKEY 64481591B98321F9
No clue how to fix this.........
Reading package lists... Done W: GPG error: http://ppa.launchpad.net trusty InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY D46F45428842CE5E
Still looking for the solution
Working Solution 2017/07
sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys D46F45428842CE5E
Solution
gpg --keyserver hkp://subkeys.pgp.net --recv-keys D46F45428842CE5E
gpg: requesting key 8842CE5E from hkp server subkeys.pgp.net
gpg: keyserver timed out
gpg: keyserver........