How do I change the privileges for MySQL user that is already created?

Standard

To list users:

select user,host from mysql.user;

To show privileges:

show grants for 'user'@'host';

To change privileges, first revoke. Such as:

revoke all privileges on *.* from 'user'@'host';

Then grant the appropriate privileges as desired:

grant SELECT,INSERT,UPDATE,DELETE ON `db`.* TO 'user'@'host';

Table:

GRANTALTER,CREATEON example TO'someuser'@'somehost'

Finally, flush:

flush privileges;

The MySQL documentation is excellent:

http://dev.mysql.com/doc/refman/5.0/en/user-account-management.html