Don't know if it would be faster, but it would look cleaner to use a Select.

Sub ChangeTest()
Dim LastRow As Long
Dim i As Long
  LastRow = Range("M" & Rows.Count).End(xlUp).Row
  For i = 2 To LastRow
    Select Case Range("M" & i)
      Case "A", "B", "C"
        Range("L" & i) = "1"
      Case "D","E","F"
        Range("L" & i) = "2"
      Case "G","H","I"
        Range("L" & i) = "3"
    End Select
  Next i

End Sub

Don't know if it would be faster, but it would look cleaner to use a Select.

Sub ChangeTest()
    Dim LastRow As Long
    Dim i As Long
      LastRow = Range("M" & Rows.Count).End(xlUp).Row
      For i = 2 To LastRow
        Select Case Range("M" & i)
          Case "A", "B", "C"
            Range("L" & i) = "1"
          Case "D","E","F"
            Range("L" & i) = "2"
          Case "G","H","I"
            Range("L" & i) = "3"
        End Select
      Next i
End Sub