How to edit MS Access database using C#

For OleDb the order of parameters is of paramount importance. The reason your update will not work, is that you are supplying the ID parameter first, whereas in your SQL it is last (comes in the WHERE clause). Move this parameter to the end and it should work.

The revised code should be something like this:

private void button1_Click(object sender, EventArgs e)
{
    string ConnString = (@"Provider=Microsoft.ACE.OLEDB.12.0;Data Source=C:\Users\Ts\Documents\testDB.accdb");
    using (OleDbConnection conn = new OleDbConnection(ConnString))
    {
        using (OleDbCommand cmd = conn.CreateCommand())
        {
            cmd.CommandText = "UPDATE [Table4] SET [EmpName] = ?, [Age] = ?, [Mobile] = ?  WHERE [ID] = ?";

            cmd.Parameters.AddWithValue("@EmpName", OleDbType.VarChar).Value = nametextBox2.Text;
            cmd.Parameters.AddWithValue("@Age", OleDbType.VarChar).Value = agetextBox3.Text;
            cmd.Parameters.AddWithValue("@Mobile", OleDbType.VarChar).Value = mobiletextBox4.Text;
            cmd.Parameters.AddWithValue("@ID", OleDbType.VarChar).Value = idtextBox1.Text;
            cmd.Connection = conn;
            conn.Open();

            int rowsAffected = cmd.ExecuteNonQuery();
        }
    }
    Form1_Load(sender, e);
    MessageBox.Show("record updated ");
}

I have made a couple of other minor changes, most importantly moving the Form1_Load and the MessageBox outside of the using block. In general you should keep code within using blocks to a minimum, so that the resources can be freed as soon as possible. I also removed the Close(). The OleDbConnection's destructor automatically calls Close(), so there is no specific need to call it yourself.

One other thing might be wrong. You have the ID as a varchar. Are you sure about this? It would be more usual if it were an integer. In which case, you would need to parse the idTextBox1.Text to an integer with int.Parse(), or safer with int.TryParse(), and change the parameter type as well.