replace a line in a text file using vb.net

Dim reader As New StreamReader(strFile)
Dim line As String = reader.ReadLine
   While line <> ""

      line = reader.ReadLine
      If line <> "" Then
         Dim values() As String = line.Split(":")
          If values(0) = "GBPtoUSD" Then
             strFile.Replace(line, "GBPtoUSD:" + txtGBPtoUSD.Text)
          End If
 
      End If

    End While
reader.Close()

strFile.Replace(line, "GBPtoUSD:" + txtGBPtoUSD.Text)