what's your db and version?
pretty much all dbs have built-in string and date functions, a
combinations of which will let you update your field to be date/datetime.
you will probably want to create a new field in the table to store the
converted dates, then check that they all have been converted correctly,
then delete the current date field and rename the new field to the old
date field's name.
using your db's string functions you will need to extract day, month and
year parts of your date text, and pass them to your db's date creation
if your db is MySQL, it has a very handy STR_TO_DATE(str, format)
function... check details in mysql ref manual.