I don't know how you handle your solr with Drupal. I program it myself with curl... But maybe this helps you on your way...
You have 2 ways:
first way: delete the old product and add it again:
$json = '{"delete":{"query":"id:' . $articleId . '"},"commit":{}}'
$url = 'http://solrhost/solr/yourcore/update/json';
Handle the url with curl, and the json string are your post fields
$json = '{"add":' . $json_of_article . ',"commit":{}}';
$url = 'http://solrhost/solr/yourcore/update/json';
Handle the url with curl, and the json string are your post fields
second way: update a field in one product
$data = json_encode(array('doc' => array('id' => $articleId, 'price' => array('set' => $newPrice))));
$json = '{"add":' . $data . ',"commit":{}}';
$url = 'http://solrhost/solr/yourcore/update/json';
I hope this helps...