Plugin should notify you about new versions automatically and update itself if you enter your Purchase Code from CodeCanyon to the plugin’s settings page.
On the image at the left you can see the look of the Purchase Code input field from the plugin’s settings page where you should add your Purchase Code if you want to enable automatic update.
If you download new plugin version from CodeCanyon please follow these steps to update from the existing version:
1. Go to your FTP and open WordPress plugins folder
2. Delete the old wpDataTables version folder
3. Upload the new version of wpDataTables
4. Go to WP-admin panel, open Plugins section
5. Deactivate wpDataTables, and then activate it again