Error when changing column type

If column type is int4 and try to change type to int2, SQL look like this:

UPDATE pg_attribute  SET atttypid=, atttypmod=-1WHERE attrelid=21459::oid AND attnum=3;

and syntax error occurs. I know that int2 is not in Data type drop down list
but error should be avoided in some way.

Regards,
Virgil