The CentOS team ( and red hat devs) backport all the security patches from the latest 2.6 kernel to the one used in CentOS 5
but you are not
using a CentOS kernel .
You are using one built by OpenVZ
so to update that one , you need to build it ( and that is not really an easy task )
At least for the first few times you build a kernel
or do not use the OpenVZ kernel ( but that might not be an option )
I would start with studying
the OpenVZ wiki
and really learn the container os that it is
and study up on building a kernel and all that needs to go into it
take a week or so and study up first
non main stream OS's are -- shall we say " fun "