change datatype in postgres

I have a table in one of my postgres databases which has a row of type varchar(5). I would like to convert it to varchar(20) data type. Is there an easy way for doing it? I`ve tried to do with this sql sentence "alter table mytable alter column mycolumn varchar(20)", and didn´t works. The table has more than 500 records. It`s something wrong on my query? Thank´s Norma Ramirez Area de Desarrollo Web norma.ramirez@tecnosoft.com.co