However, if you configured that setting before WordPress 3.5, you will see two additional boxes. Newer versions of WordPress simply hide them if they are left blank. And that leaves us with chicken/egg problem: you cannot change it until you already changed it once before.
Fortunately, web interface is not the only way to change settings in WordPress. We can go directly to MySQL and change settings there.
Of course, adjust database name and path according to your needs. My path is a bit weird as I keep WordPress files in subdirectory, but SQL commands look something like this:
# mysql -e "UPDATE wordpress.wp_options SET option_value='../content/media' WHERE option_name='upload_path';"
# mysql -e "UPDATE wordpress.wp_options SET option_value='/content/media' WHERE option_name='upload_url_path';"
Now you can refresh admin interface and everything will be in place.