I just went through this. I found two different documents. Here are the steps I did from my history:
sudo rm /usr/share/keyrings/mattermost-archive-keyring.gpg
curl -o- https://deb.packages.mattermost.com/repo-setup.sh | sudo bash -s mattermost
sudo rm /etc/apt/sources.list.d/mattermost_stable.list
sudo rm /etc/apt/trusted.gpg.d/mattermost.gpg
(as root) sudo curl -s https://deb.packages.mattermost.com/pubkey.gpg | gpg --dearmor > /usr/share/keyrings/mattermost-archive-keyring.gpg
(as root) echo "deb [signed-by=/usr/share/keyrings/mattermost-archive-keyring.gpg] https://deb.packages.mattermost.com stable main" > /etc/apt/sources.list.d/mattermost.list```