This is a ready-to-use function to replace the exact positions given by Start and Length parameters:

Function ReplacePositions(Expression As String, Start As Integer, Length As Integer, Replace As String) As String
  ReplacePositions = Mid(String:=Expression, Start:=1, Length:=Start - 1) _
                   & Replace _
                   & Mid(String:=Expression, Start:=Start + Length)
End Function

This will give:

Sub Test_Mid()
Dim starting_text As String
Dim replacement_text As String

starting_text = "Long Starting Text"
replacement_text = "End"

starting_text = ReplacePositions(starting_text, 6, 8, replacement_text)
MsgBox (starting_text) ' will show "Long End Text"

End Sub