How to update main class widget from page in Flutter

you can give callback to from child widget to parent widget to update data, you can use inherited widget or provider

You can use the Provider Pattern:

import 'package:flutter/material.dart';
import 'package:provider/provider.dart';

class ProviderTest with ChangeNotifier {
  String amount = '';

  void updateAmount(String newAmount) {
    amount = newAmount;
    notifyListeners();
  }
}

class EditInvoice extends StatefulWidget {
  @override
  _EditInvoiceState createState() => _EditInvoiceState();
}

class _EditInvoiceState extends State<EditInvoice> {
  @override
  void initState() {
    super.initState();
  }

  PageController pageController = PageController(
    initialPage: 0,
    keepPage: true,
  );

  Widget buildPageView() {
    return PageView(
      controller: pageController,
      children: <Widget>[
        AddItems(),
      ],
    );
  }

  @override
  Widget build(BuildContext context) {
    var providerTest = Provider.of<ProviderTest>(context);

    return Scaffold(
      backgroundColor: Colors.white,
      floatingActionButton: FloatingActionButton(
        onPressed: () {},
        backgroundColor: Colors.orange[600],
        elevation: 2.0,
        child: Icon(
          Icons.check,
          color: Colors.white,
        ),
      ),
      floatingActionButtonLocation: FloatingActionButtonLocation.centerDocked,
      appBar: AppBar(
        iconTheme: IconThemeData(
          color: Colors.black, //change your color here
        ),
        automaticallyImplyLeading: false,
        toolbarHeight: 140.0,
        centerTitle: true,
        title: Column(
          children: <Widget>[
            Row(
              children: <Widget>[
                IconButton(
                    onPressed: () {
                      Navigator.pop(context);
                    },
                    icon: Icon(Icons.arrow_back)),
                Padding(padding: EdgeInsets.only(left: 30.0)),
                Align(
                  alignment: Alignment.center,
                  child: Text(
                    providerTest.amount.toString(),
                    style: TextStyle(color: Colors.black),
                  ),
                )
              ],
            ),
          ],
        ),
        backgroundColor: Colors.white,
      ),
      body: buildPageView(),
    );
  }
}

class AddItems extends StatefulWidget {
  @override
  _AddItemsState createState() => _AddItemsState();
}

class _AddItemsState extends State<AddItems> {
  @override
  Widget build(BuildContext context) {
    var providerTest = Provider.of<ProviderTest>(context, listen: false);

    var items = Padding(
        padding: EdgeInsets.only(left: 20.0, right: 20.0),
        child: TextFormField(
          onChanged: (value) {
            providerTest.updateAmount(value);
          },
          decoration: InputDecoration(
            labelText: 'Amount',
            border: OutlineInputBorder(
              borderRadius: BorderRadius.all(Radius.circular(5)),
            ),
          ),
          style: TextStyle(
            color: Colors.black,
            fontWeight: FontWeight.bold,
            fontSize: 15,
          ),
          cursorColor: Colors.black,
          keyboardType: TextInputType.phone,
        ));

    return Container(
      child: Card(
          child: Padding(
              padding: EdgeInsets.only(top: 10, bottom: 5.0, left: 10.0, right: 10.0),
              child: SingleChildScrollView(
                  child: Column(mainAxisSize: MainAxisSize.min, crossAxisAlignment: CrossAxisAlignment.start, children: <Widget>[
                Text(
                  'Add Items',
                  style: TextStyle(fontWeight: FontWeight.bold, color: Colors.black),
                ),
                items,
              ])))),
    );
  }
}

And at main.dart

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return ChangeNotifierProvider(
      create: (context) => ProviderTest(),
      child: MaterialApp(
        home: EditInvoice(),
      ),
    );
  }
}

More info about Provider Pattern here:

Provider Pattern