I'm trying to upgrade from Ubuntu 14.04 to Ubuntu 16.04.3. I tried from software updater but first I get "Failed to download repository information" and I click on Ok and I get "Some software couldn't be checked for updates". So I tried to update from terminal with "sudo apt-get update" and I am getting these errors:
W: GPG error: http://dl.google.com stable Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 1397BC53640DB551
W: Failed to fetch http://dl.google.com/linux/chrome/deb/dists/stable/main/binary-i386/Packages 404 Not Found [IP: 172.217.17.174 80]
E: Some index files failed to download. They have been ignored, or old ones used instead.