We're talking about Linux Mint Debian Edition users also getting an upgrade to the Cinnamon desktop, just like users of Linux Mint 22. Assuming you're currently running Linux Mint 21.3, the update probably won't be enabled for you quite yet. You could do a clean install to get it, but IMO that would be kinda silly. Just be patient.

Just sudo apt update and upgrade

Actually, I'm just gonna ask - are you using the main edition - Linux Mint 21 - or are you using LMDE? You know Mint has two editions right, the main one based on Ubuntu and another Debian-based one?

Because if you're using the main edition, and only that would make sense since you're asking about upgrading to 22, this post isn't about you. It's about users of the Debian edition also getting some nice updates today. Anyways, there will be official upgrade instructions posted for you in the coming days on Mint's website.

If you are using the Debian edition (in which case why are you asking about 22?), then this post is about you, and yes, a simple sudo apt update followed by sudo apt dist-upgrade is all that's required. It's not a whole new version (LMDE 7 won't be out for at least another year and some change).