If its a Ticket purchased at Counter you can visit the PRS counter and give a requisition for cancellation...If its E-ticket and is W/L after chart preparation it will be cancelled automatically and refund amount will be credited back to your account in 3-4 days time.
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Also FYI TDR filing is only applicable if in case of E-tickets in RAC/Partial W/L situations after chart preparation.TDR is not applicable for Counter tickets.