Issues with /wp-admin/ directory

I’m trying to upgrade my version of WordPress. In place of the ‘Proceed’ button there is this error message: Fatal error: Call to undefined function wp_unslash() in D:\Hosting\2434477\html\thederekperkins\wp-admin\includes\file.php on line 1103.

I’ve also noticed there are various other error messages throughout WordPress that all link to the /wp-admin/ directory.

I re-uploaded the entire /wp-admin/ directory to see if that would work out any kinks, but no luck.

Is there anything else I can try to resolve these error messages or would doing a complete manual re-install be my best bet?