Drupal 7 how can I alter page content generated with module?
You can change the variables of the page content implementing the template_preprocess_page() function in your module or theme. As of Clive's comment, I want to update that you can preferably use hook_page_alter(), which is called earlier than preprocess function.

function YOURMODULE_preprocess_page(&amp;$variables) {
  // Do your changes here
  // If have Devel module installed
  // You can use dpm() function to print the contents of $variables array. 
  // dpm($variables)
}


or

function YOURMODULE_page_alter(&amp;$page) {
  // Do your changes here
  // If have Devel module installed
  // You can use dpm() function to print the contents of $variables array. 
  // dpm($page)
}


Additional Reference:


Render Arrays in Drupal 7 See the section "Creating Content As Render Array"



  A page callback should return a render array, as should hook_block_view()'s $block['content']. This allows your module and other modules to treat the content as data for as long as possible in the page generation process.