How to alter the search api views exposed form field value?
I have found an answer. I am sharing it here so that it can help someone.

Instead of form_alter hook, we can use hook_views_pre_view.

Here is the code

/**
 * Implements hook_views_pre_view().
 */
function mymodule_views_pre_view(&amp;$view, &amp;$display_id, &amp;$args) {
  if ($view-&gt;name == 'my_view_name') {
    $address = 'some address';
    $latlong = somefunction($address);
    $view-&gt;get_exposed_input();
    $view-&gt;exposed_input['latlon'] = $latlon;
    $view-&gt;execute().
  }
}