Is it possible to update/upgrade CentOS version on the server running Plesk? Or do Plesk update it?
Plesk does not do any release upgrades for OS.
In case of CentOS, it supports only package updates in the same release.
In this case, it is possible to update CentOS 6 version up to 6.9 and CentOS 7 up to 7.4 by running 'yum update'
Anyway, it is recommended to create a server backup before updating.