How can I edit loginitems.501.plist with Xcode?

Hello,

I do no longer want to use a menu bar app, but I assume it had become incompatible with Sierra and I can't stop it from launching on login. With "Find any file" I found all related files it had created and deleted them, but there is one file /private/var/db/com.apple.xpc.launchd/loginitems.501.plist, in which it left some lines, which can't be removed via Xcode. It says:

“loginitems.501.plist” is locked for editing and you may not be able to save your changes. Do you want to unlock it? “loginitems.501.plist” is currently locked because you are not the owner of the file and do not have write permission.

and then

The file “loginitems.501.plist” could not be unlocked. Could not add write permission to the file because you do not own it. Try modifying the permissions of the file in the Finder or Terminal.

I tried to change the file's permissions via "Get Info" (so that Admins can Read & Write), and also of the folder in which it is, but I still can't edit it. How can I do it?

Many thanks.