If a counter ticket, go to any counter across India with the ticket and request for change. They may additionally ask for an original photo ID of one of the passengers or/and a written request.
If an E ticket, take a print of the ticket, an Original ID of one of the passengers and request at any of the reservation counters across India. somewhere, they may ask for a written application too.
BP is change prospectively and can not be retrospect.
Can...
more... be changed till not less than 24 hrs. remain upto scheduled departure of the train.