Please rebuild your update tables in Joomla->Extensions->Manage->Update sites. After that click on the same page on "Clear Cache" and afterwards on "Find updates".
In case Acymailing still won't update you can download the latest version from your account (assuming you have a valid Essential or Enterprise subscription) and install that one.
That having said you also could consider to move on to v7. That's where the furue of Acymailing is.