If you are using sqlplus, you can alter the session and set NLS_DATE_FORMAT to modify the format, but RMAN has no such statement to run, it follows the OS setting, so you must set NLS_DATE_FORMAT on OS-level for RMAN to use.
$ NLS_DATE_FORMAT='yyyy/mm/dd hh24:mi:ss'; export NLS_DATE_FORMAT
$ rman target /
> set NLS_DATE_FORMAT=yyyy/mm/dd hh24:mi:ss
> rman target /
Please notice that, there must be no single quote to set the environment variable in Windows platform, otherwise, you will receive an error message in RMAN.