Is there any (known) way for end users to edit a Laravel session variable?
They would have to run code (something like Session::put('key', 'value');) on your server.. or take advantage of possible vulnerabilities on your application.
If you're using the Session carefully on your side (validation, etc.) it should remain the same unless you update it.
More on Laravel's Session here: http://laravel.com/docs/4.2/session