The official word from Corcom (http://www.cor.com/pdf/M.pdf
1. Open cover, using small blade screwdriver or similar tool.
2. Set aside cover/fuse block assembly.
3. Pull voltage selector card straight out of housing, using indicator pin.
4. Orient selector card so that desired voltage is readable at the bottom.
5. Orient indicator pin to point up when desired voltage is readable at bottom (note that when indicator pin is fixed, successive voltages are selected by rotating the card 90° clockwise
6. Insert voltage selector card into housing, printed side of card facing forward toward IEC connector and edge containing the desired voltage first.
7. Replace cover and verify that indicator pin shows the desired voltage.
And, of course, replace the fuse with a new one of the appropriate rating.