If you want to access your database remotely as SYSDBA or SYSOPER you’ll have to use password file authentication. Adding users to the password file is done when you grant sysdba or sysoper privilege to the desired user.
grant sysdba/sysoper to user;
If oracle password file is deleted or lost you can re-create with the following command
orapwd file="file name" entries="max. number of users"
Password file name is platform-specific.
Oracle password file required locations:
If you want to remove password from password file for specific user, then just revoke sysdba/sysoper privilege from the same user (Oracle automatically will update the password file).
More information you can find in the Oracle documentation