Steps taken to update or reschedule the scheduled jobs:
a) If we have to schedule an apex job through salesforce UI, we have to go through
Setup -> Develop -> Classes
You will see a "Schedule Apex" button. You can set up the timing from there.
b) After clicking the button "Schedule Apex",fill the Job Name,the Apex class for which we want to schedule job
and fill the Schedule apex execution in which we will go through Frequency which may be either weekly or monthly,
there will be start Date and end Date for the duration of Apex job execution and also fill the Preferred Start Time.
c) After filling all the Parameters of Scheduling jobs,click on the save button if you want to schedule otherwise cancel the process.
d) Now Suppose According to the question,if we have scheduled a job and want to change the End date of the scheduled job.
we have to follow these steps:
i)Go to setup -> search Scheduled jobs in quick search menu ->
ii> go to Manage action of your scheduled job and change the parameters you want to change and click on the save button
to reschedule the schedued job.
Hope this answer suits your question well.