サンプルマクロは、C:\Sampleフォルダ内の拡張子.txtのファイルの拡張子
を.htmに変換します。

Sub Sample()

    Dim myOldName As String
    Dim myNewName As String
    
    Const myOldDil As String = "*.txt"
    Const myNewDil As String = ".htm"
    Const myDir As String = "C:\Sample\"

    myOldName = Dir(myDir & myOldDil)
    
    While myOldName <> vbNullString
        myOldName = myDir & myOldName
        myNewName = Left(myOldName, Len(myOldName) - Len(myOldDil) + 1) & myNewDil
        FileCopy myOldName, myNewName
        Kill myOldName
        myOldName = Dir()
    Wend

End Sub