I messed up with functions.php and can't access my blog

Hi:

I tried to add a shortcode to my functions.php file following a website tip (here, in Spanish). When I saved the file I got a Syntax error and since then I have not been able to access my site.

I couldn’t fix it this morning because I have a little baby, but I did some research in Google and I downloaded Filezilla and rewrote my functions.php file (I kept the theme files) to no avail : (((

I think I have modified WP’s functions.php file and not the theme’s but I’m not sure.

My main problem is how to overwrite the file with Filezilla, I’m a newbie to this.

Thanks in advance.