sql - Update date format in mysql -

i want update date field in table. getting date value form mm/dd/yy i.e(05/06/13) format. want date convert y-m-d (i.e 2013-05-06) format , update it. used date_format date not updating .

update tablename set    coldate = date_format(str_to_date(coldate,'%m/%d/%y'),'%y/%m/%d') 

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