Change or remove topic word from url

i want to change the "topic" word from url for example like this:

from:

website.com/topic/7/the-subject/

to:

website.com/res/7/the-subject/

the second question can we totally remove topic word from url? the first question is urgent. i try to change it from forum_urls.php and rewrite_rules.php but gives not found error. Thanks.