Use hook_module_implements_alter for changing hooks execution order.

hook_module_implements_alter

/**
 * Implements hook_module_implements_alter().
 */
function mymodule_module_implements_alter(&$implementations, $hook) {
  // We need this hook running at the end.
  if ($hook == 'form_alter' && isset($implementations['mymodule'])) {
    $mymodule = $implementations['mymodule'];
    unset($implementations['mymodule']);
    $implementations['mymodule'] = $mymodule;
  }
}