Updating a wp_postmeta field not working in wordpress

I cannot update a wp_postmeta field called 'alive_days' and making its value equal to zero, in wordpress. I have tried all the two following solutions but still not working:

1. $alive_days='alive_days';$mypostid=$_REQUEST['pid'];
2. global $wpdb;
3. $wpdb->query("update $wpdb->wp_postmeta set meta_value==\"$ziro\" where post_id=\"$mypostid\" and meta_key=\"$alive_days\"");

1. $ziro=0;
2. $mypostid=$_REQUEST['pid'];
3. update_post_meta($mypostid, 'alive_days',$ziro);

The field value will not change after the execution.

via Chebli Mohamed