I found following thread at : https://issuetracker.google.com/issues/133299031#comment14
Hello, In-app update priority of the release can be set using the Play Developer Publishing API's Edits methods. There is a new 'inAppUpdatePriority' field under Edits.tracks.releases. The documentation does not mention the new field yet but you should still be able to set it. In-app update priority can not be set from the Google Play Console at the moment.
I am using google-api-php-client with Service Account authentication, I would like to ask how to set 'inAppUpdatePriority' using google-api-php-client I have tried following in my PHP code.
$publisher->edits_tracks->update(self::PACKAGE_NAME, self::EDIT_ID, 'production', new \Google_Service_AndroidPublisher_Track);