Topic: Change permalink to category/post

Hi, I would like to change your permalink structure: from post/postname to category/postname.

I tried to change the string in url.class.php from

return $url.'post/'.$post['slug'].'/';

to

return $ url.strtolower ($ post ['category']). '/'. $ post ['slug']. '/';

but it doesn't works.

I also do not know how to modify the .htaccess file.

Can someone please help me? Thanks!