Switch in bottomsheet not changing status on Flutter

You need to put the switch in its own StatefulWidget and use a callback to return the value to your main class. A bottom sheet uses a different context so calling setState in your example above doesn't work.

class _MyHomePageState extends State<MyHomePage> {
  bool _switchValue = false;

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        body: Center(
          child: RaisedButton(
            child: const Text('SHOW BOTTOM SHEET'),
            onPressed: () {
              showModalBottomSheet<void>(
                context: context,
                builder: (BuildContext context) {
                  return BottomSheetSwitch(
                    switchValue: _switchValue,
                    valueChanged: (value) {
                      _switchValue = value;
                    },
                  );
                },
              );
            },
          ),
        ),
      ),
    );
  }
}

class BottomSheetSwitch extends StatefulWidget {
  BottomSheetSwitch({@required this.switchValue, @required this.valueChanged});

  final bool switchValue;
  final ValueChanged valueChanged;

  @override
  _BottomSheetSwitch createState() => _BottomSheetSwitch();
}

class _BottomSheetSwitch extends State<BottomSheetSwitch> {
  bool _switchValue;

  @override
  void initState() {
    _switchValue = widget.switchValue;
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return Container(
      child: CupertinoSwitch(
          value: _switchValue,
          onChanged: (bool value) {
            setState(() {
              _switchValue = value;
              widget.valueChanged(value);
            });
          }),
    );
  }
}

Use StatefulBuilder to change status of switch inside BottomSheet

    showModalBottomSheet(
     context: context, builder: (BuildContext context) {  
      return Container(  
       child: StatefulBuilder(  
        builder: (BuildContext context, StateSetter stateSetter) {  
         return CupertinoSwitch(  
           value: _switchValue,  
           onChanged: (val) {  
             stateSetter(() => _switchValue = val); 
           },);  
         },  
       ),  
      )  
   });