Hi Jiexi
If you have the Swiss Travel Pass for 3 days in a row: The period of validity for the activated days always begins at 00.00 hrs on the first day of validity and ends at 05.00 hrs on the 3 day.
If you have the Swiss Travel Pass for 3 days Flex: The period of validity for the chosen, activated day begins at 00.00 hrs and ends at 05.00 hrs on the following day.
Already activated travel days can be deactivated/changed as often as desired until the start of the validity period at 23:59.
An activated day pass can no longer be deactivated/changed on the day of travel.
More info here: My switzerland
Have a nice day. EllenvS.