Question:

Why is it not possible to edit Apex scheduled jobs? A Save button does not exist in the Edit page of Apex scheduled jobs to save the updated change. 

Example:

Answer:

This is a limitation in Salesforce. The end-users have to delete an old scheduled job and then create a new one to make a change.

Related Documentation:

Salesforce Help Documentation: Apex Scheduler - Existing jobs need to be editable and show scheduling details