> so Winhttrack does not support to update just a
> small portion of a whole project, does it? an update
> performed must be from top to toe?
There is no way to know what part of a web site has or has not changed.

> I wonder where's the "do not erase already
> downloaded file" option(found in 3.40 update) gone.

No where: <http://www.httrack.com/html/step9_opt5.html> Do not purge old files.
(But remember they will not be linked into the mirror)