1.) Open “single-portfolio.php” and look for this line of code:

<dd><?php echo $att_date; ?></dd>
And then replace it with this one:

<dd><?php echo date('d/m/Y', strtotime($att_date)); ?></dd>

2.) Open “functions.php” and look for this line of code:
<dd><?php echo $att_date; ?></dd>
And then replace it with this one:

<dd><?php echo date('d/m/Y', strtotime($att_date)); ?></dd>