Save the splitted word into original place of file when edited

There are multiple lines and multiple words in a line

i have managed to split single line from multiple lines into richtextbox one by one through button click and managed to split words from splitted line into boxes.

but i am not able to save the splitted line and splitted words on the original file when the line/word is edited with the

other lines/words which are not modified.

below is my code:

Public Class Form1

    Dim currentline As Integer = 0

    Public Enum Direction
        Up = -1
        Down = 1
    End Enum

    Private Sub BtnUp_Click(sender As Object, e As EventArgs) Handles BtnUp.Click
        SelectLine(Direction.Up)
    End Sub

    Private Sub BtnDown_Click(sender As Object, e As EventArgs) Handles BtnDown.Click
        SelectLine(Direction.Down)
    End Sub

    Private Sub SelectLine(direction As Direction)

        Dim lineindex As Integer = currentline + direction
        If lineindex < 0 Then
            'this is the first line! 
            Exit Sub
        End If

        If lineindex > RichTextBox1.Lines.Count - 1 Then
            'this is the last line! 
            Exit Sub
        End If

        Dim searchedtext = RichTextBox1.Lines(lineindex)
        Dim indexofText As Integer = RichTextBox1.Find(searchedtext, RichTextBoxFinds.MatchCase)
        RichTextBox1.Select(indexofText, searchedtext.Length)
        RichTextBox1.Focus()

    End Sub

    Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
        Dim lines As String() = {"ABCDE", "123456", "EFGHI", "78910"}
        Me.RichTextBox1.Lines = lines
        Me.ActiveControl = RichTextBox1
    End Sub

'************CODE TO SAVE FILE*****************

    Private Sub Button3_Click(sender As Object, e As EventArgs) Handles Button3.Click

        Dim saveMe As New SaveFileDialog()

        saveMe.Filter = "Text Documents(*.txt)|*.txt"

        If saveMe.ShowDialog() = Windows.Forms.DialogResult.OK Then

                        File.WriteAllLines(saveMe.FileName, RichTextBox1.Lines)

            MsgBox("File Saved as : " + saveMe.FileName)

        Else

            MsgBox("Failed to pick up file to save as")

        End If

    End Sub

    '************CODE END SAVE FILE*****************

    Private Sub RichTextBox1_SelectionChanged(sender As Object, e As EventArgs) Handles RichTextBox1.SelectionChanged
        Dim fci As Integer = RichTextBox1.GetFirstCharIndexOfCurrentLine()
        currentline = RichTextBox1.GetLineFromCharIndex(fci)
        Dim s As String = String.Format("Current line: {0}", currentline)
        Me.Text = s
        Me.BtnUp.Enabled = currentline >= 0
        Me.BtnDown.Enabled = currentline < RichTextBox1.Lines.Count
    End Sub 

End Class