Get the latest version
Download it from http://www.roundcube.net/download and copy the Tarball to a directory on your server.
Unpack the tarball and read UPGRADING and INSTALL files and check system requirements of the new version.
cd <where-the-tarball-is-saved> tar xf roundcubemail-*.tar.gz
Update your existing Roundcube installation
The easiest way to do this is to use the installto.sh shell script bundled with Roundcube.
cd <the-unpacked-roundcube-directory> bin/installto.sh <your-existing-roundcube-directory>