No standard format. You may write, as you wish.
Dated _____________________
To
The chief Reservation Supervisor,
----------------------- PRS
Reg:...
more... PNR #
Sub: Change in Boarding Point
Dear Sir,
Enclosed please find PRS ticket in original/ printout of e-ticket from ................... Station to ................... Station.
I have to board this train from ................... Station.
You are therefore requested to change the boarding station to .................station.
Thanking you.
Yours faithfully
-------------------
(Any one of the passengers in above PNR
along-with ID proof in original)