Read the whole file into an array, replace one of the elements of that array, then write the whole file again. Text ends up the same - vb.net

Question

I have a form3 that when the user exits it attempts to save the time they spent playing. When trying to replace the line I end up with the exact same text, therefore nothing changes in the file. Why will it not replace the text? Thanks in advance.
File.AppendAllLines(Application.StartupPath + "\content\main\userdata\tgametime.txt", FileLines)
'this line will not write to file if it has the same file name because FileLines is still using it, if this could be solved too awesome!
Private Sub SaveGameTime()
Dim fullpath = Path.Combine(Application.StartupPath + "\content\main\userdata\gametime.txt")
Dim FileLines = File.ReadLines(fullpath)
Dim alltext = File.ReadAllText(fullpath)
Dim datalist = New List(Of String)
Dim line
datalist.Add(tbGameName.Text + " : " + LBtimeout.Text)
TextDiag(FileLines.Count.ToString, "Clear")
If FileLines.Count > 0 Then
If alltext.Contains(tbGameName.Text) Then
For Each line In FileLines
TextDiag(line, "Clear")
line.Replace(line, tbGameName.Text + " : " + LBtimeout.Text)
TextDiag("new line: " + line, "Clear")
Next
File.AppendAllLines(Application.StartupPath + "\content\main\userdata\tgametime.txt", FileLines)
Else
TextDiag("Line not found, adding new line", "Clear")
File.AppendAllLines(fullpath, datalist)
End If
Else
TextDiag("No lines not found, adding new line", "Clear")
File.AppendAllLines(fullpath, datalist)
End If
datalist.Clear()
fullpath = Nothing
FileLines = Nothing
datalist = Nothing
TextDiag Output

Answer 1

Got it working, replaces text at a specified string value in a file formatted like this: "Text > 0 : 0 : 0" . It gets a new time value extracted from a label formatted like this: "0 : 0 : 0", removes everything but the numbers and colons to this: "0:0:0", formats them to a TimeSpan and adds them to the outputlines which also contain any other original text.
Thank you
Dim fullpath = "yourpath"
Dim FileLines = File.ReadLines(fullpath)
Dim outputlines As New List(Of String)
Dim alltext = File.ReadAllText(fullpath)
Dim datalist = New List(Of String)
Dim line
Dim lbtime = TimeSpan.Parse(LBtimeout.Text.Replace(" ", ""))
datalist.Add("youstring" + " : " + "yourstring")
If FileLines.Count > 0 Then
If alltext.Contains("yourstring") Then
For Each line In FileLines
If line.contains("yourstring") Then
Dim liness = line.substring(line.indexof(">") + 1)
Dim linerep = liness.replace(" ", "")
Dim linetime = TimeSpan.Parse(linerep)
Dim timesum As TimeSpan = lbtime + linetime
line.replace(line, "")
line = "yourstring" + " > " + timesum.ToString
outputlines.Add(line)
Else
outputlines.Add(line)
End If
Next
File.WriteAllLines("yourpath", outputlines)
Else
File.AppendAllLines(fullpath, datalist)
End If
Else
'No lines not found, adding new line
File.AppendAllLines(fullpath, datalist)
End If
datalist.Clear()
fullpath = Nothing
FileLines = Nothing
datalist = Nothing