どう探すか、は、とりあえず置いといて…
curl -O http://www.ijs.si/software/amavisd/amavisd-new-<version>.tar.gz
tar zxvf amavisd-new-<version>.tar.gz
cd amavisd-new-<version>

まずは、ひたすらコピーしてしまう。
cp -p amavisd /usr/local/bin/
cp -p amavisd-agent /usr/local/bin/
cp -p amavisd-nanny /usr/local/bin/
chown root /usr/local/bin/amavisd*
chmod 755 /usr/local/bin/amavisd*

cp -p amavisd.conf /etc/
chown root:amavisd /etc/amavisd.conf
chmod 640 /etc/amavisd.conf

次は、amavisd.confの書き換え。慎重に、地道に。