Correct method to transition from snapshots to milestones

I’ve updated to openHAB 3.0 (quite happy so far) and have been installing snapshots. As I’m running in a live environment and development has progressed I’d like to move back to milestone updates.
Per the download page I revised the repository path:

echo 'deb testing main' | sudo tee /etc/apt/sources.list.d/openhab2.list

But am getting the following when doing a system update:

E: The repository ' testing Release' no longer has a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.

Am I missing something?

There is an issue with the repository that keeps resurfacing for some reason.
You can refer to the steps here:

Thanks @candiesdoodle, that seemed to work…I think. When installing the update it seemed to error (below) but the console now indicates that I have M3 installed:

pkg: error processing package openhab (--configure):
 installed openhab package post-installation script subprocess returned error ex
it status 1
Processing triggers for systemd (245.4-4ubuntu3.3) ...
Errors were encountered while processing:

Oh yeah, thats another error. you need to have openhab in stop state for the install script to finish properly. so next time before updating just run sudo systemctl stop openhab.service
Nevertheless, if you get similar error you can run this immediately after the error sudo dpkg --configure openhab
You can do it now also but stop the openhab service first