Change block titles from h2 to h3
As a rule, we should never make changes in core files. If you will read the readme.md file in omega theme, you will see the following,


  Omega 4.x is a base theme and as such should never be used or
  modified directly. Instead, you should use the sub-theming
  functionality provided by Drupal.
  
  The easiest way to create a new Omega-based sub-theme is by using
  Drush. Omega comes with a very
  easy-to-use Drush command for generating sub-themes through a
  command-line wizard. You can start the sub-theme creation wizard by
  invoking drush omega-wizard in the command-line.


So you should first create omega sub-theme which is quite easy process and then make changes in sub-theme's block.tpl.php file.