If you already have an existing GNO installation running v2.0.4 or earlier, then there is no automated upgrade mechanism. Instead, upgrades must be done manually.
Before doing such an upgrade, ensure that you have a backup of your existing system.
Although it is more disk space intensive, you should upgrade from your existing v2.0.4 system by first installing the current distribution into an otherwise unused directory hierarchy (a ``from scratch'' install). You should then review each component of your old distribution and move any necessary files into the new hierarchy. It is strongly recommended that you make maximal use of the /usr/local hierarchy so that future upgrades are easier. See the section in this Guide on Planning for Future Upgrades for details.
This method of upgrade is tedious, but in the end it makes for a cleaner GNO distribution and easier future upgrades.