Example update user meta ACF

update_users_meta_acf.php

$user_id = wp_get_current_user()->ID;

update_user_meta( $user_id, 'first_name', $_POST['user_firstname'] );
update_user_meta( $user_id, 'last_name',$_POST['user_lastname'] );
update_user_meta( $user_id, 'linkedin', $_POST['user_linkedin'] );
update_user_meta( $user_id, 'facebook', $_POST['user_facebook'] );
update_user_meta( $user_id, 'instagram', $_POST['user_instagram'] );
update_user_meta( $user_id, 'twitter', $_POST['user_twitter'] );
update_user_meta( $user_id, 'biography', $_POST['user_description'] );
update_user_meta( $user_id, 'education', $_POST['user_education'] );

//json of [specialties, subspecialties]
$bb_specialties = json_decode(stripslashes($_POST['user_specialty']));
            
update_user_meta($user_id, 'bb_specialties', count( $bb_specialties ), false);
            
$i = 0;
            
foreach( $bb_specialties as $bb_sp) {
                
  update_user_meta($user_id, 'bb_specialties_'.$i.'_specialty', $bb_sp->specialty, false);

  update_user_meta($user_id, 'bb_specialties_'.$i.'_subspecialty', $bb_sp->subspecialty, false);
                
  $i++;
            
}