Syntax:
ALTER {DATABASE | SCHEMA} [db_name]
    alter_specification ...

alter_specification:
    [DEFAULT] CHARACTER SET [=] charset_name
  | [DEFAULT] COLLATE [=] collation_name

ALTER DATABASE enables you to change the overall characteristics of a
database. These characteristics are stored in the db.opt file in the
database directory. To use ALTER DATABASE, you need the ALTER privilege
on the database. ALTER SCHEMA is a synonym for ALTER DATABASE as of
MySQL 5.0.2.

The CHARACTER SET clause changes the default database character set.
The COLLATE clause changes the default database collation.
http://dev.mysql.com/doc/refman/5.0/en/charset.html, discusses
character set and collation names.

You can see what character sets and collations are available using,
respectively, the SHOW CHARACTER SET and SHOW COLLATION statements. See
[HELP SHOW CHARACTER SET], and [HELP SHOW COLLATION], for more
information.

If you change the default character set or collation for a database,
stored routines that use the database defaults must be dropped and
recreated so that they use the new defaults. (In a stored routine,
variables with character data types use the database defaults if the
character set or collation are not specified explicitly. See [HELP
CREATE PROCEDURE].)

The database name can be omitted, in which case the statement applies
to the default database.

URL: http://dev.mysql.com/doc/refman/5.0/en/alter-database.html

