The only way to set the OwnerId is to be logged in as that user.
Per the CronTrigger Docs only query is supported and you cannot use DML for this object.
The simplest way to do this is to first log in as the right user and just delete the job from setup (setup->monitor->scheduled jobs), and then reschedule (setup->develop->apex classes, click 'Schedule Apex').
You could do this with apex as well if you have the scheduled job id
Id jobId = '00000000000000';
String jobName = 'JobName';
String cronString = '0 0 0 3 9 ? 2022';
system.schedule(jobName, cronString, new SchedulableClass());