Can’t do manual reinstall – “permission denied” on some WP files

I am trying to manually reinstall WordPress (with FTP) to cure problems with the dashboard not rendering.

Some files in my wp-admin and wp-includes folders cannot be deleted or over-written — when I try to delete or over-write these files, the error message I see is “553: Can’t open that file: Permission denied”.

The files that I can’t modify seem like a random list. Some of the files I can’t touch are class-simplepie.php and default-widgets.php in wp-includes, all of the contents in wp-includes/Text and its subfolders, load-scripts.php and load-styles.php in wp-admin, theme-install.php in wp-admin, and about half of the files in wp-admin/js.

The file permissions on all these files are the same as on the files I can delete or over-write.
I’ve disabled all plugins except Akismet, but that doesn’t seem to have made any difference.

Any ideas on what’s going on, and how I can resolve the problem???