update_field

Prototype: update_field($selector, $value, [$post_id]);

Note: to update a date field, the data has to be formatted first (even in case of copying the data from a date field).

$date_from = get_field( 'old_field', $old_post_id );
$date = DateTime::createFromFormat( 'd/m/Y', $date_from);
$date = $date->format( 'Ymd' );

update_field('new_field', $date, $new_post_id);

Source : https://www.advancedcustomfields.com/resources/update_field/