change data type from text to numeric

Hi everyone,

sorry to bother you with a simple question, (I'm a new user of postgresql),
how do I change the data type of a column of text, to numeric or integer?

I tried with:

ALTER TABLE table_name
ALTER COLUMN col_name TYPE integer
;

It gives me a mistake
ERROR: la columna no puede convertirse automáticamente al tipo integer
SUGERENCIA: Especifique una expresión USING para llevar a cabo la
conversión.

translation: the column can't be converted automatically to type integer
.... specify an expression using USING to make the conversion.

I also tried on pgadmin3, but couldn't found how to do it...

thanks,
Daniel