package update

I'm running Linux Ubuntu and tried to update my packages using the
update.package() command. It appeared to download the updates ok but I got
the following message:

The downloaded packages are in ?/tmp/RtmpFM82Ry/downloaded_packages?
Warning in install.packages(update[instlib == l, "Package"], l, contriburl =
contriburl,  :
  'lib = "/usr/lib/R/site-library"' is not writable
Error in install.packages(update[instlib == l, "Package"], l, contriburl =
contriburl,  : 
  unable to install packages
Calls: update.packages -> install.packages

What does this mean ? And more importantly, how do I address it ?