Hi,
Generally the previous CumulusMX "program" version will continue to work without issue. However, as CumulusMX is in constant development for new stations, sensors, and features, periodic program upgrades are published and that will trigger the (red) CMX Upgrade notice. It is a manual process to upgrade the CumulusMX program.
Latest version download zip is at
https://cumuluswiki.org/a/Software#Latest_build_distribution_downloadFor most upgrades it is as simple as:
With CMX stopped, copy all the files and folders from the new unzipped download and paste into the existing \CumulusMX folder so to over-write existing.
Restart CMX, and that should be it.
Of course, as always MAKE SURE you have a good full backup in the event of any power or issue during the upgrade.
Enjoy,
Paul