Source: As worded by Indian Railways, Government of India

Yes, you can. All you have to do is submit a written request to the Station Manager of the original boarding station or contact any Computerized Reservation Center, at least 24 hours before the departure of the train. However, no refund is permissible for the unused portion of the journey. The change of station once made will be final i.e., in case a passenger has changed the boarding point, he/she will loose all rights to board the train from the original boarding point. If found traveling without any proper authority to travel, he/she will have to pay fare with penalty between original boarding point to changed boarding point.