SF_14.0, SF_14.1, SF_14.2, SF_14.3 - https://www.progress.com/documentation/sitefinity-cms/api-widget-operations

UpdateThePropertiesOfAWidgetRB.json

{
   "propertyValueGroup": {
      "ComponentId": "c9cd38c3-3a8c-44e4-a9dd-ed3d81579724",
      "Properties": [
         {
            "Name": "Content",
            "Value": "Hello world edited"
         }
      ]
   }
}