We have changed magento "code pool" starting with Cart Reservation v1.5. This means that in order to update your Cart Reservation extension, you will need to erase all old Cart Reservation files before copying new version.
Please note that cart reservation tables should not be erased.
Here is the complete list of files and folders that should be deleted:
Or just use attached PHP script to remove old files(Just upload it to your magento root folder and execute from your browser e.g. http://www.yourdomain.com/upgrade_cart_reservation.php) - File attached.
P.S. If you decide to use automated script to delete all old files - make sure you have required user permissions to remove files. Additionally you must receive "Success!" message after the script is executed