Aftter the update of all files and try to login to the backend (http://mysite/serendipity_admin.php) perhaps you get the alert "username or password wrong"
Solution: check the serendipity_config_local.inc.php
edit $serendipity['versionInstalled'] = 'x.x.x'; to your previous version then call login-page again
now you have the upgradepage if not than check file permissions.