I've checked and the latest version of Aimersoft Video Converter Ultimate for Mac is 5.7. If you're having issues updating from within the program's interface, then I recommend checking the official website. Simply save the installation package with the latest version and overwrite your current installation. Don't worry about registration as it will be kept safely even if you do the overwrite process.
Access the official website, choose Mac OS as the operating system and initiate the download procedure.