Setting permissions for wordpress

I installed wordpress on lamp and cannot update themes or install plugins. I get an “unable to create directory” error when I try to. When I spoke with support they told me to do this: root@domain:~# chown -R www-data /var/www/wordpress

but what I got was this: chown: cannot access `/var/www/wordpress’: No such file or directory

So, I was told to find where wordpress was installed but my understanding is that according to the tutorials here on DO its installed in home and then moved to /var/www

Can anyone help with this?

Thanks!