If you are already running an older version of Nagios
you can upgrade it anytime you just need to download the latest tar archive of it and configure it
[[email protected] ]# service nagios stop
[[email protected] ]# wget http://prdownloads.sourceforge.net/sourceforge/nagios/nagios-4.2.0.tar.gz
[[email protected] ]# tar -zxvf nagios-4.2.0.tar.gz
[[email protected] ]# cd nagios-4.2.0
[[email protected] ]# ./configure
[[email protected] ]# make all
[[email protected] ]# make install
[[email protected] ]# service nagios start