This error is encountered when you attempt to manually upgrade the plugin while a copy of the plugin is still installed on your server. 

Before installing the new plugin, the old version must be disabled and deleted.  This will not affect any existing member/product data or settings on your website, as these are stored in your site database, and deleting the plugin files only affects files stored in the /wp-content/plugins folder.

All the steps for a general manual upgrade can be found here.