If you have encountered this error while trying to upgrade to v2.3.0, you'll find all the steps you need to take in the article Download and Manual Upgrade Instructions for MemberMouse v2.3.0
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.