When purchasing an add-on you will receive a download link for a .zip file. This file can be installed through the Plugins > Add New page by clicking on Upload Plugin.
Important: if you’re downloading the .zip file with Safari you might end up with an already unzipped archive. Try using another browser (like Chrome or Firefox) or just create an archive of the extracted folder contents again.
WP Recipe Maker Premium is an add-on to WP Recipe Maker, so you just keep the free version installed as well. Installing the add-on will not change any of your existing recipes.