Sub Test_Mid()
'Replace ("1011112", "1", "2", 5, 1) выдаёт 212, а надо 1011212
Dim strText As String
strText = "1011112"
Mid(strText, 5, 1) = "2"
Debug.Print strText
End Sub

1011212

'---------------------------------------------------------------------------------------
' Procedure : fnReplace
' Author    : Sonnih
' Date      : 26.12.2016
' Purpose   : Ïîäìåíà ñòàíäàðòíîé ôóíêöèè Replace
'---------------------------------------------------------------------------------------
'
Public Function fnReplace(ByVal Expression, ByVal Find, ByVal Replacement, _
        Optional ByVal Start As Integer = 1, Optional ByVal Count As Integer = -1, _
        Optional ByVal Compare As VbCompareMethod = vbBinaryCompare) As String

   On Error GoTo fnReplace_Error

    If IsNull(Expression) Then
        fnReplace = ""
        Exit Function
    End If

    If IsNull(Find) Then
        fnReplace = Expression
        Exit Function
    End If
    
    If IsNull(Replacement) Then
        fnReplace = Expression
        Exit Function
    End If

    If Len(Expression) < Start Then
        fnReplace = Expression
        Exit Function
    End If
   
    If Count < -1 Then
        fnReplace = Expression
        Exit Function
    End If
   
    If Start <= 1 Then
        fnReplace = Replace(Expression, Find, Replacement, Start, Count, CompareMethod)
        Exit Function
    End If
    
    If Start > 1 Then
        fnReplace = Left(Expression, Start - 1) & Replace(Expression, Find, Replacement, Start, Count, CompareMethod)
        Exit Function
    End If
      
   On Error GoTo 0
   Exit Function

fnReplace_Error:

    MsgBox "Error " & Err.Number & " (" & Err.Description & ") in procedure fnReplace"

End Function