Can I change the Job Number on a Job?

The short answer is no this is not possible if the Job is already created.

The Job number is sequentially set per Job that is created according to the numbering sequence set under Settings > Job Settings.

If the job has not been created yet and you want to set the next number for the system to use including the Job prefix, then go to Settings > Job Settings and set the numbering sequence in the Job Numbers tab.