give read&write permission to only 1 file in folder

Question

I am trying to manually modify preferences file located at

/Library/Preferences/SystemConfiguration/preferences.plist

To modify it, I need to give it read&write permission. I did that. The problem is that the file is in a folder, for which I cannot change permission (I am admin and I have no access), nor do I want to change permissions to the whole folder for security reasons. I only want to change it for preferences.plist file. How can I achieve this?

These are the pop-ups I get when trying to modify it.

Upon modifying permissions to parent folder I get this:

Answer 1

To allow user_name writing to preferences.plist add an ACL with:

sudo chmod +a "user_name allow write" /Library/Preferences/SystemConfiguration/preferences.plist

This changes the output of:

ls -laeO /Library/Preferences/SystemConfiguration/preferences.plist
-rw-r--r--  1 root  wheel  - 59715  9 Feb 23:25 /Library/Preferences/SystemConfiguration/preferences.plist

to:

ls -laeO /Library/Preferences/SystemConfiguration/preferences.plist
-rw-r--r--+ 1 root  wheel  - 59715  9 Feb 23:25 /Library/Preferences/SystemConfiguration/preferences.plist
0: user:user_name allow write

but doesn't change the permissions of the superior folder or other files residing in the same folder:

drwxr-xr-x  15 root      admin        -             510 24 Mär 17:23 SystemConfiguration

To remove the ACL later simply enter:

sudo chmod -a "user_name allow write" /Library/Preferences/SystemConfiguration/preferences.plist