How to change Search API index field machine name?
My answer I found in search_api_solr module search_api_solr.api.php file. I used hook_search_api_solr_field_mapping_alter
/**
 * Change the way the index's field names are mapped to Solr field names.
 *
 * @param SearchApiIndex $index
 *   The index whose field mappings are altered.
 * @param array $fields
 *   An associative array containing the index field names mapped to their Solr
 *   counterparts. The special fields 'search_api_id' and 'search_api_relevance'
 *   are also included.
 */
 function hook_search_api_solr_field_mapping_alter(SearchApiIndex $index, 
                                                               array &amp;$fields) {
   if ($index-&gt;entity_type == 'node' &amp;&amp; isset($fields['body:value'])) {
    $fields['body:value'] = 'tm_text';
   }
 }

Here you can add required Solr index field name('tm_text') for according field('body:value').