1. Option:  Use the [markup module][1] to add custom HTML code.

 2. Option: Write a [custom template file][2] node--type.tpl.php where type is the machine name of your content type. 

 3. Make a custom module and use hook_node_view. Here is [an example][3].


  [1]: https://www.drupal.org/project/markup
  [2]: https://www.drupal.org/docs/7/theming/template-theme-hook-suggestions
  [3]: http://drupal.stackexchange.com/questions/10204/modify-node-in-hook-node-view-for-drupal-7#16620