The upgrade instructions are available at Oro documentation website.
The current file describes significant changes in the code that may affect the upgrade of your customizations.
*.class
parameters for all entities were removed from the dependency injection container. The entity class names should be used directly, e.g.'Oro\Bundle\EmailBundle\Entity\Email'
instead of'%oro_email.email.entity.class%'
(in service definitions, datagrid config files, placeholders, etc.), and\Oro\Bundle\EmailBundle\Entity\Email::class
instead of$container->getParameter('oro_email.email.entity.class')
(in PHP code).