Imports System

Public Module Module1

    Public Sub Main()

        'Your String here

        Dim strChange = "some text --/ this will be changed ++/ some text"

        'Get the index of - character

        Dim startindex As Integer = strChange.IndexOf("-")

        'Get the index of + character

        Dim endindex As Integer = strChange.IndexOf("+")

        'use substring method to get part of characters between --/ and ++/

        Dim strpart As String = strChange.Substring(startindex+4, (endindex - startindex-5) )

        'Replace the string with new value

        strChange = strChange.Replace(strpart, "This is new text")

        Console.WriteLine(strChange)

    End Sub

End Module