This tool will allow you to update a hosted feature service (either an ArcGIS Online hosted feature service, or a Portal for ArcGIS hosted feature service) using a shapefile/feature class as the input. If a unique field is specified, the existing features in the hosted feature service will be deleted that contain this same value in the feature class. If a unique field is not specified, all of the features from the shapefile/feature class will be appended to the hosted feature service.
Note: it's recommended to create a backup of your hosted feature service before executing this tool.