Place from where I can download individual files

While working on includes/languages/english/classic/header.php file, I messed up and want to download the file once again. Could someone please help me the place from where I can download individual files? Thanks in advance.

There is no place I know of for that. (Maybe you can get just one file from github, not sure.) But you should have a complete set of clean files saved anyway, for future comparison use. Download the full fileset from the link on the homepage, copy the file you need, and store the zip or the unzipped set (or both) somewhere safe.

Also, you should not be working on (changing) any of the /classic/ files. Instead, you should copy all /classic/ folders and their files to new folders (call them /classic2/ if you don't want to personalize them), so that you retain an untouched Classic template. This is useful for debugging - you can switch from the template you are working on to Classic and see if the problem goes away. If it does, the issue is in a template file; if not, it is elsewhere.

Thanks!