Drupal-7: Apply database updates

Every so often, a database gets restructured. When that happens, Drupal needs to be told to make the changes. It's always a good idea to make a backup before doing this. If you are using YSite, it's as easy as clicking /admin/config/y/site There are two ways to apply the database updates: using Drush, or using Drupal:

Apply database updates using Drush

Any of the following work:
drush updb
drush updatedb

Apply database updates using Drupal7 / YAD7

Log in as admin. Then go to /update.php If you can not log in as admin, then you will have to temporarily disable the admin requirement in the settings.php by changing the line that says:
$update_free_access = FALSE;
to
$update_free_access = TRUE;
After this, do the update, and then change the line back again. It is very bad to leave this line in the latter state.