bbpress change the word forum, topic, reply in the forum to another word I choose

You can hook to translation filters of WordPress to change any word or phrase. These filters are: gettext, ngettext and gettext_with_context:

add_filter('gettext', 'change_bbpress_wording', 10, 3);
add_filter('ngettext', 'change_bbpress_wording', 10, 3);
add_filter('gettext_with_context', 'change_bbpress_wording', 10, 3);

function change_bbpress_wording ($translated, $untranslated, $domain) {

    if ($domain == 'bbpress') {

        $translated = str_ireplace('Forum', '*desired word*', $translated );
        $translated = str_ireplace('Topic', '*desired word*', $translated );
        $translated = str_ireplace('Reply', '*desired word*', $translated );

    }

    return $translated;
}

Note that we use str_ireplace which is case-insensitive, for case-sensitive replacement use str_replace instead.

See gettext filter hook codex page for more examples.