How to upgrade MySQL?

New versions of MySQL are released periodically, and you can upgrade from one version of MySQL to a newer version. Upgrading information is provided in the MySQL manual at dev.mysql.com/doc/refman/5.0/en/upgrade. html. However, there are special considerations when upgrading. As a precaution, it is wise to back up your current databases, including the grant tables in the mysql database, before upgrading. MySQL recommends that you do not skip versions. If you want to upgrade from one version to a version more than one version newer, such as from MySQL 4.0 to MySQL 5.0, you should upgrade to the next version first. After that version is working correctly, you can upgrade to the next version.
And so on. In other words, upgrade from 4.0 to 4.1, then from 4.1 to 5.0. Occasionally, incompatible changes are introduced in new versions of MySQL. Some releases introduce changes to the structure of the grant tables. For instance, MySQL 4.1 changed the method of encrypting passwords, requiring a longer password field in the grant tables. After upgrading to the newer version, you should run the mysql_upgrade script. It checks your files, repairing them if needed, and upgrades the system tables if needed. Before MySQL version 5.0.19, the mysql_upgrade script does not run on Windows; it runs only on Unix. On Windows, you can run a script called mysql_fix_privileges_tables with MySQL versions prior to 5.0.19. The script upgrades the system tables but does not perform the complete table check and repair that mysql_upgrade performs.