The ALTER TABLE statement syntax is explained in the manual at:
To put it simply, there are two ways you can alter the table to use a new character set.
1. ALTER TABLE tablename DEFAULT CHARACTER SET utf8;
This will alter the table to use the new character set as the default, but as a safety mechanism, it will only change the table definition for the default character set. That is, existing character fields will have the old character set per column. For example:
mysql> create table mybig5 (id int not null auto_increment primary key,
-> subject varchar(100) ) engine=innodb default charset