ie.
shell:appsFolder\Microsoft.FlightSimulator_8wekyb3d8bbwe!App -DeleteCloudSaves
But be prepared to loose all past saved data, ie Logbook, achievements, custom settings etc etc
You can also backup (when MSFS is not running), most of this data, by saving the WGS folder (subfolders & Files).
Restoration is a little tricky, as you have to fool MSFS to use the restored WGS folder data, and not that Data saved on the cloud.
I had to do this (Successfully) yesterday, after corrupting one of the files in this folder structure, with an incorrect manual edit.
Moral: Always make a backup, before editing files !!