If updating from alpha1, alpha2 or older dev version visit /core/install.php if you get a 500 error. This is happening because the "InitSubscriber" was removed. Clear cache.
Template files were renamed! If you have custom template files they will need to be renamed to match the new names. Example: photos_albumlinks.html.twig is now photos_album_links.html.twig. Then clear cache.
This release includes several important changes for sites using FPP objects in Panelizer displays and are using an editorial workflow module like Workbench Moderation, Revisioning or Workflow. See the change notices for full details.