As I already mentioned in my comment - your job doesn't run at 6 am every day, it runs every 21 hours.
As a second remark, you should seriously consider switching to DBMS_SCHEDULER - it's so much nicer than DBMS_JOB.
Anyway, to let this job run at 9am every day, this should do the trick:
DBMS_JOB.CHANGE (
job => your_job_id,
interval => 'trunc(sysdate) + 1 + 9/24');