How to update Drupal core and contrib modules.

Install Read Only Mode module (https://www.drupal.org/project/readonlymode). In this way, people still have access to the content, but there is no risk that the content is changed while you make the changes.

1. Clone your live site foo.com to upc.foo.com in the existing, old platform
2. Migrate upc.foo.com to the new platform (the new version of Drupal), without renaming it, and make sure it works
3. Update modules with "drush pm-update --no-core --no-backup"
4. Test everything in the updated upc.foo.com to make sure it works fine
5. Clone your live site foo.com to old.foo.com (for recovery purposes)
6. Migrate the updated upc.foo.com to foo.com (this effectively renames the site)