Select For Update statement in PostgreSql

I want to select fields in a record of a table and update just one of these fields. How can I do?

I try this:

SELECT v.idvideo, v.title 
FROM video v  WHERE v.schedulingflag IS FALSE AND v.errorflag IS FALSE 
ORDER BY v.idvideo  LIMIT 1 FOR UPDATE ;

UPDATE video  SET schedulingflag = true;

But in this way it sets field "schedulingflag" true in all record!