Here is a good guide from ArcGIS Runtime SDK for Qt on Edit Features, https://developers.arcgis.com/qt/latest/qml/guide/edit-features.htm
And there is a sample in AppStudio Desktop, called "Update Attributes" that is based on this sample, https://developers.arcgis.com/qt/latest/qml/sample-code/update-attributes-feature-service/
I hope this is useful,
Erwin