How to update single value in multi dimensional Post Meta?

You may need to unserialize the data to get the array and loop through it thus:

$userid = 832; // or 1540
$votes = get_post_meta($postid,'voter');
$votes = maybe_unserialize($votes);

if (is_array($votes)) {
    // votes is the array, key is numeric index, vote is subarray
    foreach ($votes as $key => $vote) {
         // subarray values are in another array with key 'voter'
         if ($vote['voter']['voter_id'] == $userid) {
             $votes[$key]['voter']['vote'] = $newvote;
             $votes[$key]['voter']['voter_ip'] = $newvoterip;
         }                 
    }
    update_post_meta($postid,'voter',$voter);
}