Plz go to the nearest Reservation Counter, with the ticket and ID proof in original, and give request letter for change of Boarding Point from Mysore to Bangalore.
Also please be note that Change of Boarding Point HAS TO BE DONE ATLEAST 24 Hrs, PRIOR to Train's departure from its ORIGINATING STATION and NOT PASSENGER BOARDING POINT.
In your case please change your Boarding Point before 21st April