The System Updates tool is enabled in Plesk by default. It is located in Tools & Settings > the Server Management section > System Updates. Alternatively, you can access it in Tools & Settings > Update Settings > the Available Updates and All Packages tabs.
Alternatively, you may manually by running Plesk installer or system update via SSH:
# apt-get update && apt-get upgrade (For Ubuntu/Debian)
# yum update (For CentOS/CloudLinux/RHEL)