Hope someone can help...

I have imported a large amount of data from a database application into a
new spreadsheet. There is a column that includes dates of birth in the format
,say, 28.07.1965 How can I format the cells in that column so that they read
in the format 28/7/1965. I have tried formatting the cells and trying
different variations but they dont seem to have any effect on the existing
data. I need to sort the data and the current formatting is not suitable.