A job can be edited in the Web Portal and the App (depending on the user's permissions).
Go into your Web Portal > Jobs > View Jobs > Edit > Save & Close 
To change the time on a job, you need to change that in the section, Scheduled Visit(s) as shown below.
To edit a job in the scheduler, click the job that you would like to edit > Edit Job > make the time changes and save / dispatch.
To edit a Job in the App you will need to go to Jobs > search the job you would like to edit > swipe left > select Edit and make the time changes > save / dispatch.
