The default place to put files for Apache, if you don't have Plesk or any other control panel type thing, is /var/www which is why the default apache page mentions it. So ignore that. It has nothing to do with anything. It is the default message that Apache displays.
i.e. your files are supposed to be where they are (e.g. in /var/www/vhosts/whatever or similar).
You need to be running as root to run websrvmngr -- so if you are not logged in as root you can't run it and that might be what is causing your problem. Use: su -
to change to root (and enter the password obviously).
However, another reason why you might see the apache test page is if ALL your domains use index.htm as their index files instead of index.html -- under certain circumstances, after a Plesk update, your httpd.conf file may get overwritten with one that does not include index.htm as as index file. You can check this very easily by entering the full
www.domain.com/index.htm rather than just
www.domain.com
If the page is displayed then that's definitely your problem and you'll need to take a look at (for redhat type distros) /etc/httpd/conf/httpd.conf (I think) and search for DirectoryIndex and add index.htm to the end of the line. You'll then need to restart apache with the command: service httpd restart (or similar depending on your distribution).
I've not figured out why you get the Apache test page in these circumstances - I'd have expected a "page cannot be displayed/file not found" type error. Nevertheless it seems to happen.
You can also stop all plesk-related services with the command: service psa stopall
and then restart them with: service psa start
In case something is just a little confused after your update and needs prodding.
I'm sure there are other things you can try too but the above would be my first ports of call.
Faris.