if (have_rows('repeater_field', $post_id)) {
$new_repeater = array();
while (have_rows('repeater_field', $post_id)) {
the_row;
// perform check
// will depend on what you're checking for
if (get_sub_field('the_sub_field_to_check') == $some_value) {
// the value is one you want to save
$new_repeater[] = array(
'sub_field_1_name' => get_sub_field('sub_field_1_name'),
'sub_field_2_name' => get_sub_field('sub_field_2_name'),
'sub_field_3_name' => get_sub_field('sub_field_3_name'),
);
} // end if check field
} // end while have rows
update_field('repeater_field', $new_repeater, $post_id);
} // end if have rows
