Sub ReplaceText()
    Dim c As Range
    For Each c In ActiveSheet.UsedRange
        c = Replace(c, "Что заменить", "На что заменить")
    Next
End Sub

Sub Main()
Dim strDirPath, strMaskSearch, strFileName As String
Dim wb As Workbook

strDirPath = "C:/123/" ' Папка поиска
strMaskSearch = "*.xls*" ' Маска поиска

strFileName = Dir(strDirPath + strMaskSearch)

Do While strFileName <> ""
    
    Set wb = Workbooks.Open(strDirPath + strFileName)

        ReplaceText
  
     wb.Close True
    
    strFileName = Dir
Loop

End Sub