As newer alpha, beta, and stable releases of Nagios Core are released, you should strongly consider upgrading as soon as possible. Newer Nagios Core 4 and other releases usually contain critical bug fixes, so its important to stay up to date. Assuming you've already installed Nagios Core from source code as described in the quickstart guide, you can install newer versions of Nagios Core easily. You don't even need root access to do it, as everything that needed to be done as root was done during the initial install.
Make sure you have a good backup of your existing Nagios Core installation and configuration files. If anything goes wrong or doesn't work, this will allow you to rollback to your old version.
Become the nagios user. Debian/Ubuntu users should use sudo -s nagios.
su -l nagios
Removed the following old HTML files that were used by the web frontend. They have been replaced by PHP equivalents.
Download the source code tarball of the latest version of Nagios Core (visit https://www.nagios.org/download/ for the link to the latest version).
Extract the Nagios Core source code tarball.
tar xzf nagios-x.y.z.tar.gz cd nagios
Run the Nagios Core configure script, passing the name of the group used to control external command file permissions like so:
Compile the Nagios Core source code.
Install updated binaries, documentation, and web web interface. Your existing configuration files will not be overwritten by this step.
Verify your configuration files. Correct any errors shown here before proceeding with the next step.
/usr/local/nagios/bin/nagios -v /usr/local/nagios/etc/nagios.cfg
Restart Nagios Core. Debian/Ubuntu users should use /etc/init.d/nagios restart.
/sbin/service nagios restart
That's it - you're done!
It shouldn't be too difficult to upgrade from Nagios 2.x to Nagios Core 4. The upgrade is essentially the same as what is described above for upgrading to newer 3 or 4 releases. You will, however, have to change your configuration files a bit so they work with Nagios Core 4:
Also make sure to read the "What's New" section of the documentation. It describes all the changes that were made to the Nagios 3 code since the latest stable release of Nagios 2.x. Quite a bit has changed, so make sure you read it over.
If you currently have an RPM- or Debian/Ubuntu APT package-based installation of Nagios Core and you would like to transition to installing Nagios Core from the official source code distribution, here's the basic process you should follow:
Note that different RPMs or APT packages may install Nagios Core in different ways and in different locations. Make sure you've backed up all your critical Nagios Core files before removing the original RPM or APT package, so you can revert back if you encounter problems.
Package-based installations of Nagios Core can be upgraded using the normal package management tools for your system. Note that the latest version available from repositories for a system may be considerably older than the latest Nagios Core source release.
Most packages will keep existing configuration files, but a back up of your critical Nagios Core files will allow you to revert back if you encounter problems.