> i downloaded a site and now i tried to update a
> single page using the following command - 
> httrack www.xyz.com/aboutus.aspx –O F:Temp -r1 –q
> –-update
> 
> But instead of updating it delete all other pages

The default is to obsolete parts of a mirror after an update.
To override this add the -X0 flag <http://httrack.com/html/fcguide.html>

If you update only one page, then links on that page will NOT be connected to
other pages in your mirror.

> winhttrack 3.42.2 version. 

Update is normally quick except for dynamic pages. It simply asks the site,
has this file changed.  I get over 5 pages/sec on dialup.

3.4x has many problems, including redownloading unchanged files during update. 
Try 3.32 <http://forum.httrack.com/readmsg/17840/17833/index.html>