I followed these instructions and overwrote the previous install.
# go into the the directory that contains your existing "piwik/" installation
$ wget http://builds.piwik.org/latest.zip # download latest version
$ unzip -o latest.zip # extract it to "piwik/" directory
$ rm latest.zip # delete the archive and read below the next step
# go into the the directory that contains your existing "piwik/" installation
$ wget http://builds.piwik.org/latest.zip # download latest version
$ unzip -o latest.zip # extract it to "piwik/" directory
$ rm latest.zip # delete the archive and read below the next step