First, it's really a bad idea to use Drupal 8 beta version on production site like me. Because Drupal 8 beta release do not provide a way to upgrade between beta versions. The database and API are not frozen at all.
If you are unlucky like me, you can try following steps:
- Download beta 10 and do a fresh install.
- Download beta 6 database through `mysqldump ----skip-add-drop-table --insert-ignore`.
- compare node* tables. There are new columns in release. We need to change insert statments to ignore new columns. When necessary, specify default value to the columns (I specify "zh-hans" to langcode).
- Import beta 6 database to Drupal 8 rc1 site.
- If you are not able to reach clear cache page, install latest drush. run `drush cr` to rebuild cache
- open the site and monitor php error.log. You will get some syntax issues. About 10 in my case. Hack relative code to ignore syntax errors.