Syntax:
ALTER FUNCTION func_name [characteristic ...]

characteristic:
    { CONTAINS SQL | NO SQL | READS SQL DATA | MODIFIES SQL DATA }
  | SQL SECURITY { DEFINER | INVOKER }
  | COMMENT 'string'

This statement can be used to change the characteristics of a stored
function. More than one change may be specified in an ALTER FUNCTION
statement. However, you cannot change the parameters or body of a
stored function using this statement; to make such changes, you must
drop and re-create the function using DROP FUNCTION and CREATE
FUNCTION.

You must have the ALTER ROUTINE privilege for the function. (That
privilege is granted automatically to the function creator.) If binary
logging is enabled, the ALTER FUNCTION statement might also require the
SUPER privilege, as described in
http://dev.mysql.com/doc/refman/5.5/en/stored-programs-logging.html.

URL: http://dev.mysql.com/doc/refman/5.5/en/alter-function.html

