Upgrade Process Example in WordPress

installer.php

<?php

/**
 * This class can be used as an installer class to install tables and such.
 * And it can be also used to make updates.
 */

class My_Plugin_Installer() {
  /**
   * Update functions for versions.
   * Have this functions defined elsewhere.
   */
  public $updates = array(
		'1.1.0'  => 'my_plugin_update_110',
    '1.2.0'  => 'my_plugin_update_120',
	);
  
  /**
	 * Check from which version we are updating.
   * 
   * @param string $from_version This version is retrieved from the option. It's still the old version because we are updating the option after the updates.
	 * 
   * @return void 
	 */
	public function update( $from_version ) {

		if ( ! defined( 'MY_PLUGIN_UPDATING' ) ) {
			define( 'MY_PLUGIN_UPDATING', true );
		}

		foreach ( $this->updates as $version => $update_function ) {
			if ( version_compare( $from_version, $version, '<' ) ) {
				 $update_function();
			}
		}

		update_option( 'giveasap_version', MY_PLUGIN_VERSION );

	}
}

// Defined elsewhere.
function my_plugin_update_110() {}

function my_plugin_update_120() {}

main.php

<?php

/**
 * This is your main plugin class (or file).
 * Or at least a file that's included when plugin is called. 
 */
define( 'MY_PLUGIN_VERSION', '1.2.0' );

add_action( 'init', 'my_plugin_check_versions' );

/**
 * This function will be called to check for a new plugin version.
 * If this is the first time we run this, the get_option will return 1.0.0 because it's the default value.
 */
function my_plugin_check_versions() {
  if ( ! defined( 'IFRAME_REQUEST' ) && get_option( 'my_plugin_version', '1.0.0' ) !== MY_PLUGIN_VERSION ) {
	$installer = new My_Plugin_Installer();
	$installer->update( get_option( 'my_plugin_version', '1.0.0' ) );
	do_action( 'my_plugin_updated', MY_PLUGIN_VERSION );
  }
}