Note: You can only cancel your ticket before the chart is prepared. Post chart preparation, you'll need to file a TDR. Please check the IRCTC Top rules below to know whether you are eligible to cancel or not:
E-Ticket can be cancelled online till preparation of charts and refund will be granted by PRS system as per extant Railway Refund Rules.
After preparation of charts, you'll have to file online TDR for claiming the refund. The refund case will be decided by Railways. The TDR should be filed by the same user who books the ticket. After receiving the refund receipt from the Railways, it shall be credited in the same account through which booking was made via the opted payment gateway.
The TDR should be filed after 30 minutes from the scheduled departure of the train. No refund shall be granted on partially confirmed E-Ticket against which none of the passengers has travelled.
If your train is scheduled to depart before or at 12:00 PM, the chart is prepared the previous night. If your train is scheduled after 12:00 PM, the chart is prepared at least 4 hours before the departure.
Also, please check this link to know the cancellation charges: Link