Solution:
This usually happens when an update was interrupted or corrupted in some way. Here’s how to fix it(usually):
Use FTP client to get to your server and delete the /wp-admin, /wp-includes directories and all files in the directory where wp-admin and wp-includes were, except for wp-config.php and any other custom files that you might have had in there.
Then upload all files manually – download the latest WP version from here, extract it to your computer and upload all files to your server.
If you don’t want to delete your old files(which is always a good idea), just create a dummy directory, for instance /~backup and move all of the files and directories there(except /wp-content/ directory and /wp-config.php file).