linux 19.3 update fault : The repository 'http://packages.linuxmint.com tricia InRelease' is not signed.

Please advice me what to do to rectify this problem:
sudo apt update && sudo apt upgrade
[sudo] password for naas:
Hit:2 http://archive.canonical.com/ubuntu bionic InRelease
Hit:3 http://security.ubuntu.com/ubuntu bionic-security InRelease
Hit:4 http://archive.ubuntu.com/ubuntu bionic InRelease
Hit:5 http://archive.ubuntu.com/ubuntu bionic-updates InRelease
Hit:6 http://archive.ubuntu.com/ubuntu bionic-backports InRelease
Err:1 http://packages.linuxmint.com tricia InRelease
403 Forbidden [IP: 91.241.95.156 80]
Reading package lists... Done
E: Failed to fetch http://packages.linuxmint.com/dists/tricia/InRelease 403 Forbidden [IP: 91.241.95.156 80]
E: The repository 'http://packages.linuxmint.com tricia InRelease' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.[/B][/B][/U]
N: See apt-secure(8) manpage for repository creation and user configuration details.