When editing existing jobs selection modify does not give me the option to modify existing job or copy.
For more information on this topic, please see the Administration Module 2-Basic Operation of NetVault Backup-Web-based Training
The pop up will only appear if the job you are modifying is using saved selection set i.e. when editing a Job target set the target set you modify should have a target set name and likewise for selection set backup option set and so on.
If you do not wish to use saved selection sets just modify the job name and modify the job as you require then save. This will copy the job with modifications and create the new job with new job name.