The temperature wheel must be removed from the unit to replace the number strip. Peel off the old strip and clean the wheel well - isopropyl alcohol works (not rubbing alcohol).
Part number for Celsius is 124 821 29 67 and Fahrenheit 124 821 34 67.
This strip will fit W123 (from MY 81), W124, W126, W107 (from MY 82) and W201.