Code:
Private Sub Worksheet_Change(ByVal words As Range)

        For Each C In Worksheets("Sheet1").Range("words").Cells
            If Right(C.Value, 2) = "ar" Then
                C.Value = Left(C.Value, (Len(C.Value) - 2)) + "áste"
            ElseIf Right(C.Value, 2) = "er" Then
                C.Value = Left(C.Value, (Len(C.Value) - 2)) + "íste"
            ElseIf Right(C.Value, 2) = "ir" Then
                C.Value = Left(C.Value, (Len(C.Value) - 2)) + "íste"
            End If
        Next
        
End Sub


Instructions:
In the worksheet select the range with all the text to be updated.
Name that range "words".

Select the worksheet's tab and rmb>view code.
Paste the above code into the window. Note: the name "Sheet1" in the code should be changed to match the name of the specific worksheet.

After that any change to a cell in the range "test" will trigger a check of the all the cells in the range "words" and make the replacements specified.
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Glenn