The documentation stating that you cannot change a coordinate in MapPolyline path directly after the path has been added to the item. And that you need to copy the path first, change the coordinate and assign the path copy back to MapPolyline path https://doc.qt.io/qt-6/qml-qtlocation-mappolyline.html#details

I tested it and you actually don't need to copy the path to update it. You can directly change the coordinate(s).

// this works
mapPolyline1.path[0] = QtPositioning.coordinate(5.0, 5.0)

// this also works
mapPolyline1.path[0].latitude = 5.0