Deleting certain parts of a text file

[larzi]

I am creating a childrens program where the user has an opportunity to change their password or delete an account. The information is inside a text file and i don't know how to delete only the password etc... At the moment my code is:

Public Function ChangePassword_Click(ByVal password As String, ByVal start As Integer, ByVal length As Integer, ByVal studentpass2 As String) As String
    studentpass1 = InputBox("Please enter your previous password.")
    studentpass2 = InputBox("Please enter your new password.")
        If start + length >= Len(password) Then
            DeleteSubString = Left(password, start - 1)
        Else
            DeleteSubString = Left(password, start) & Mid(password, start + length + 1)
        End If
        InsertSubString = Left(password, start - 1) & studentpass2 & Mid(password, start)
MsgBox ("You have successfully changed your password"), vbExclamation
End Function
I have also tried this:
Private Sub cmdchangepassword( ) Click
Function (byVal username as String, ByVal password as String, ByVal studentpass1 as String, ByVal studentpass2 as String) as String
    Replace (password, studentpass1, studentpass2
End Function

Nothing is working!!!