Repository changes its ‘Suite’ value from ‘testing’ to ‘stable’
While apt update you may get repository no longer has a release file error, it happens when a repository owner changes its suite value or version. Do not panic if this is the first time you are encountering this kind of issue.
Simply allow the release information to get changed by adding below small flag after apt update.
apt update --allow-releaseinfo-change
Once the above command run successfully you can run upgrade command.