I live in Övermark/Ylimarkku (Övermark is more used becourse of it's mainly a swedish speaking village, but Ylimarkku is more used on the net), and when I checked the postal code 64610 it comes to Övermark in Lappajärvi.
How can I change it?
The postal codes are not yet editable. At the moment it is a separate database and in the long run we don't want to maintain two databases. We want to merge them with the main database and make them editable together with the other toponyms. For some countries we begun to add the postla codes as alternatenames for the pseudo language 'post'.
So the best at the moment would be, as you already done, to add the information in Swedish and Finnish to the main GeoNames table.