Manual:Update.php

checks if the MediaWiki database schema need to be updated in order to work with the current version of MediaWiki. You should run  after each update of MediaWiki or after the update or install of an extension that requires a schema update (such as CheckUser). Most extensions that require schema updates will say so in their documentation.

From the browser
Upgrading docs describe how to run the update script if you do not have access to the command line.

From the command line
From the command line, or an SSH shell or similar, change to the maintenance directory and execute the update script: $ php update.php

may exit with errors, leaving the database in an inconsistent state. Always backup the database before running the script!

Parameters
This script offers several parameters (generic, script dependant and script specific) which may be required in some wiki environments.

Generic maintenance parameters

 * : used to get a prompt of all available script parameters (generic, script dependant and script specific) together with short explanations of their usage. These are less often used parameters.

Script specific parameters

 * : skip countdown sequence
 * : do not purge objectcache table. Normally update.php purges the objectcache table in the database (generally contains the parser cache and the messages cache), if you don't want to purge this table, use the --nopurge option.
 * : Update shared tables as well.
 * : Run this script even if it is disabled by configuration options.
 * : Skips compatibility checks

Extensions can install/update their schemas using LoadExtensionSchemaUpdates hook.