How do I update a row in the datagridview in form1 from textboxes in form2

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C#

DataGridView

hi new to c#
i have created a datagridview in form one. When i double click a row in the datagridview, the program populates form2 with all the information in that row. How do i update the datagridview in form1 from form 2? I also cannot get the rowindex in form2.

Any help will be appreciated.

FORM1cs

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using Microsoft.VisualBasic;

namespace Datagridview1
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        DataTable table = new DataTable();

        private void Form1_Load(object sender, EventArgs e)
        {
            //' Add columns to your datatable, 
            // with the name of the columns and their type 

            table.Columns.Add("Id", Type.GetType("System.Int32"));
            table.Columns.Add("First Name", Type.GetType("System.String"));
            table.Columns.Add("Last Name", Type.GetType("System.String"));
            table.Columns.Add("Age", Type.GetType("System.Int32"));

            // Add rows to the datatable with some data

            table.Rows.Add(1, "XXXX", "YYYYY", 21);
            table.Rows.Add(2, "SSDD", "hGSQ", 33);
            table.Rows.Add(3, "fgfgd", "jgfdd", 53);
            table.Rows.Add(4, "cvfghyghj", "sdrgtyh", 19);
            table.Rows.Add(5, "hghfd", "ghjgdf", 36);
            table.Rows.Add(6, "cvvdfgh", "juyrfdvc", 63);

            dataGridView1.DataSource = table;
        }

        public void btnInsert_Click(object sender, EventArgs e)
        {
           
        }
        int index;
        private void dataGridView1_CellClick(object sender, DataGridViewCellEventArgs e)
        {
            // get the index of the selected datagridview row
            index = e.RowIndex;

            DataGridViewRow selectedRow = new DataGridViewRow();

            // display data from datagridview selected row to textboxes

            selectedRow = dataGridView1.Rows[index];

            textBox1.Text = selectedRow.Cells[0].Value.ToString();
            textBox2.Text = selectedRow.Cells[1].Value.ToString();
            textBox3.Text = selectedRow.Cells[2].Value.ToString();
            textBox4.Text = selectedRow.Cells[3].Value.ToString();
            label6.Text = index.ToString();       



        }
        private void btnUpdate_Click(object sender, EventArgs e)
        {
            // the new row
            DataGridViewRow newDataRow = new DataGridViewRow();
            // get data from textboxes to the row
            newDataRow = dataGridView1.Rows[index];
            newDataRow.Cells[0].Value = textBox1.Text;
            newDataRow.Cells[1].Value = textBox2.Text;
            newDataRow.Cells[2].Value = textBox3.Text;
            newDataRow.Cells[3].Value = textBox4.Text;
        }
     

        private void btnDelete_Click(object sender, EventArgs e)
        {
            index = dataGridView1.CurrentCell.RowIndex;
            dataGridView1.Rows.RemoveAt(index);
          
        }

        public void dataGridView1_CellDoubleClick(object sender, DataGridViewCellEventArgs e)
        {
           
            index = e.RowIndex;                                //      Add THIS TO POPULATE THE SECOND FORM
            Form2 pos = new Form2();
            DataGridViewRow selectedRow = new DataGridViewRow();
            selectedRow = dataGridView1.Rows[index];
            pos.textBox5.Text = this.dataGridView1.CurrentRow.Cells[0].Value.ToString();
            pos.textBox6.Text = this.dataGridView1.CurrentRow.Cells[1].Value.ToString();
            pos.textBox7.Text = this.dataGridView1.CurrentRow.Cells[2].Value.ToString();
            pos.textBox8.Text = this.dataGridView1.CurrentRow.Cells[3].Value.ToString();
            
            pos.ShowDialog();

        }    

      
        }
    }

FORM2cs
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace Datagridview1
{
    public partial class Form2 : Form
    {
        public Form2()
        {
           
            InitializeComponent();
            Form1 frm1 = new Form1();
        

        }
        int index;
        private void dataGridView1_CellMouseClick(object sender, DataGridViewCellMouseEventArgs e)
        {
           index= e.RowIndex;
            label1.Text = index.ToString();
          
            //  index = e.RowIndex;
            Form1 frm1 = new Form1();
            // the new row
            DataGridViewRow newDataRow = new DataGridViewRow();
            // get data from textboxes to the row
            newDataRow = dataGridView1.Rows[index];
            newDataRow.Cells[0].Value = textBox1.Text;
            newDataRow.Cells[1].Value = textBox2.Text;
            newDataRow.Cells[2].Value = textBox3.Text;
            newDataRow.Cells[3].Value = textBox4.Text;

        }
       
      
      
        public Form2(DataGridView dataGridView1)      // ADD THIS
        {
           
          //  DataGridViewRow row = (DataGridViewRow)dataGridView1.Rows[e.RowIndex];
        }

        private void Delete_Click(object sender, EventArgs e)
        {
            index = dataGridView1.CurrentCell.RowIndex;
            dataGridView1.Rows.RemoveAt(index);
        }
       // int index;
       
     //  public int index { get; }
    
        private void Update_Click(object sender, EventArgs e)
        {
         
            DataGridViewRow newDataRow = new DataGridViewRow();
            newDataRow.Cells[0].Value = textBox1.Text;
            newDataRow.Cells[1].Value = textBox2.Text;
            newDataRow.Cells[2].Value = textBox3.Text;
            newDataRow.Cells[3].Value = textBox4.Text;




        }
        DataGridView parentDGV= new DataGridView();  // THIS IS IMPORTANT
        public DataGridView dataGridView1;
    }

   

}

What I have tried:

hi
i have tried almost everything spend over 5 days searching the net.
have tried as per code above.