Date manipulation

Hello,
I have dates in the format 25-05-2009, in some excel file and I have formatted them to show as May-2009.

Now i need to actual value of these cells to become May-2009 and remove the old data that were showing as 25-05-2009. how do I do that quickly since i have many hundreds of dates like that and it would be easier for me the sort them if only months and years are showing in the cell