How to upgrade Tomcat on Plesk?
According to Plesk Onyx Software Requirements, Apache Tomcat 5.5 - 8.0, depending on the Linux distribution, is supported.
It means that Plesk supports the Tomcat Version officially provided and supplied by OS vendor and is updated from official repositories.
Note: While Tomcat 8 is presented in Debian 8, official support for it is yet to be implemented in Plesk. Refer to Does Plesk support Tomcat 8?
If Tomcat upgrade is required, refer to the official Tomcat documentation.
Note: Tomcat upgrade is performed at your own risk.