Emacs: Failed to verify signature archive-contents.sig
See https://emacs.stackexchange.com/questions/233/ for the same question.
The signature check failed because you don't have the new key (the old signature key expired on Sep 23). The new key is available from the usual GPG key-servers, comes with Emacs≥26.3, and can also be obtained by installing the package gnu-elpa-keyring-update.
Try renaming your ~/.emacs.d/elpa/gnupg file to something else as a backup and then run M-x package-refresh-contents. This is what worked for me.