Once you have purchased the premium plugin, you will receive an e-mail with a link to download the zip file, containing the plugin. Save this in a folder on your computer.
Then you go to your website, click ‘Plugins’, ‘Add new’ and ‘Upload’. You now select the zip file that you have just downloaded.
Click ‘activate’. A message will now ask you to enter your license key in order to use the plugin.
You have received this license key via e-mail.
Once you have entered the license key, you are ready to work with the plugin, and future updates will also happen via wp-backend.