Only updating part of a column

I would like to know if I can update part of a column. ie

update myfile set substr(direct_key,1,10)='1234567890' where
substr(direct_key,1,10)='0987654321';

This does not work. Is this possible? Or am I looking at this the wrong
way?

--
Dave Smith
Candata Systems Ltd.
(416) 493-9020
dave@candata.com