If you hold a ticket booked on internet, take the ticket and a valid identity card (in case of e ticket), go to any PRS with a request to change your boarding point.
If you hold a paper ticket (booked from reservation office), then the rule says the change is to be done at the station where the ticket was purchased from. In case you are not able to do it at the station where you purchased ticket, walk into any PRS and try your luck.