You need to visit PRS counter [ reservation counter at station] for the same.
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The request for change of Boarding Point should be submitted at-least 24 hours before the scheduled departure of the Train from the station.
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Fill...
more... up a reservation form and enter your details and PNR no. with the new boarding station clearly mentioned.
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Approach supervisor at the nearest Railway Reservation Office with with a copy of your e-ticket and the actual id [in original] mentioned therein.
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This facility will be given once only.