I have used Ubuntu for many years (about 10 years on this computer). I have upgraded from release to release with no problems in the past. This time, the upgrade from 18.04.6 LTS to version 20.04 fails. I have three types of failure; two with Software updater and one with terminal.
Type 1 with Software updater: When I try to upgrade I get "Failed to download repository information" "Check your Internet connection." This problem is intermittent. Occasionally, it succeeds. All other applications have no problems reaching the Internet. My Internet connection is very reliable with 10Mbps or more download speeds.
Type 2 with Software updater: When I get a repository download and request an upgrade, I start the upgrade process but it fails on step 2 "Setting new software channels" and then it reverts to the current version.
Type 3 with Terminal:
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: http://archive.canonical.com/ubuntu precise Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40976EAF437D05B5
W: Failed to fetch http://archive.canonical.com/ubuntu/dists/precise/Release.gpg
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40976EAF437D05B5
W: Some index files failed to download. They have been ignored, or old ones used
instead.
Any suggestions? I assume I am not pulling information from the proper repository but I don't know how to direct my computer to the right place. Perhaps it is a different issue.
Any help is appreciated.