If you just need the Date and not the Time, this format - 2008-02-19 - works. If your input is as you show (mm/dd/yyyy/h:mm am/pm), you could use a Find and Replace with a regex. The regex would be:
(\d{1,2})\/(\d{1,2})\/(\d{4})(\/.*)
It captures mm as $1, dd as $2, and yyyy as $3. Then, the replacment pattern would be:
$3-$1-$2
Note that the last parenthetical - (/.*) - probably isn’t necessary, but it creates a 4th capture group that ensures the last capture group that you care about - (\d{4}) - doesn’t pick up any stray garbage that would cause the resulting value to not be in the format that is expected.