Hi apapenfuss,
This article seems to be referencing an old version of the plugin. However, the file it references is still in the same location in the new version, but the overwrite file you should create is now in a different location.
If you are successful in finding the file then create a new file named /themes/your-theme/tribe-events/pro/widgets/mini-calendar-widget.php and copy the contents from /plugins/events-calendar-pro/views/widgets/mini-calendar-widget.php
I can’t stress enough how important it is to avoid the Plugins Editor in the WP Dashboard and instead use a text-editor and FTP client. One tiny error can bring the whole site down so at the very least I would make sure you have FTP access before making changes.