It is just like any other WordPress plugin, go to WordPress plugins page and click update now link:
And to enable automatic update, you need to login to your wpdownloadmanager.com from
Admin Menu >> Downloads >> Settings >> Updates:
However, you also may follow the following steps ( specially when upgrading from free to pro ):
From the next update, you will be able to use the automatic upgrade as usual.
For any technical issue, if you are already using pro version please post in pro forum and free version users please post in free forum. Otherwise, if you have any pre-sale or order related query please contact live chat support team. For technical support.