The change the definition of a db column, the query below can be used for example, if we have this db schema

users (
    firstname varchar(20),
    lastname varchar(20),
    age char(2)
)

To change the type of age column from char to int, we use the query below:

ALTER TABLE users CHANGE age age tinyint UNSIGNED NOT NULL;

General format is:

ALTER TABLE table_name CHANGE column_name new_column_definition