So basically it's referring to airings of the series when it was a special.

It's all segments from the first season. (Although there are some segments missing)
What you are referring to in terms of "last September" is the fact that the show originally started as a special hosted by Raymond Burr, Karl Malden and finally Robert Stack. It wasn't originally intended to be a weekly series. I have the old specials on DVD converted from old VHS tapes.

They updated them by going through later episodes that had episodes on the cases. They announced they in the press release. The cases would be updated. If the stories were solved, it would be mentioned. The series frequently went back and reaired old stories with updates.

Yes, I was thinking the same exact. It's suppos d to be Season 1 and they're referencing a mystery from "last September."

I'm assuming you're referring to what Amazon Prime is currenty airing. But yeah, when the very first episode of "Season 1" had an update on a mystery previously aired I was like HUH? How can there be an "update" if this is E1 S1?! Something is definitely array here lol