How do I delete PHPSESSID on client computers

I am working with PHP sessions on my website.

The session path was /folder, later on I changed to / to fit the new purpose.

Now, old users cant login.

It seems they now have two PHPSESSIDs stored on their browsers - one with path /folder and the other /.

What can I do to ensure that old users can login while ensuring that the session is sitewide with "/".