Can’t change file permissions of 3 files

Hey,
unfortunately, the file permissions of three files (index.php, admin-ajax.php, users.php) were st to 200. I was told to set them back to 604, but unfortunately, this doesn’t work. I can’t edit the file permissions from files that are in /wp-admin for example. Files in wp-content work. When I try to change them, Filezilla tells me, that I don’t have the permission to do this. Now I can’t access the site(I can still access wp-admin).
Thank you very much!