having problems upgrading to 4.2.1

I’m trying to upgrade to 4.2.1 but i keep getting this message can anyone help me i didn’t have this problem before.

Downloading update from https://downloads.wordpress.org/release/wordpress-4.2.1-no-content.zip…

Unpacking the update…

The update cannot be installed because we will be unable to copy some files. This is usually due to inconsistent file permissions.: wp-admin/includes/update-core.php

Installation Failed