Like most things, SQL, it depends on which DBMS/DBD you are using. But update is definitely not the right command -- that's for updating values. In standard SQL the command to change the column structure of a table would be ALTER TABLE, but see the docs for your DBMS.
Just to extend a little bit. Basically there are two types of database "languages" - DDL and DML. DML (Data Manipulate Language) is a set of statements that can be used to operate on the data, for example insert/delete/update. DDL (Data Definition Language) is a set of statements used to operate on the definition of database components, for example create/alter.