Hello.
When I change a file or directory permission then how can I reset it?

Thank you.

There is no system default. The default is what you decide it should be for that directory or file. If you are working with ACLs though, you might look at the -b option for setfacl to see about clearing the settings each time.

When you create a file or directory, the default file permissions assigned to the file or directory are controlled by the user mask. The user mask is set by the umask command in a user initialization file. You can display the current value of the user mask by typing umask and pressing Return.
The user mask contains the following octal values:
The first digit sets permissions for the user
The second sets permissions for group
The third sets permissions for other, also referred to as “world”
Note that if the first digit is zero, it is not displayed. For example, if umask is set to 022, 22 is displayed.
To determine the umask value you want to set, subtract the value of the permissions you want from 666 (for a file) or 777 (for a directory). The remainder is the value to use with the umask command. For example, suppose you want to change the default mode for files to 644 (rw-r--r--). The difference between 666 and 644 is 022, which is the value you would use as an argument to the umask command.
You can also determine the umask value you want to set by using the following table, which shows the file and directory permissions that are created for each of the octal values of umask.