How to change Java heap size for Tomcat 9 installed as Windows Service from command line?

I have Tomcat 9 installed as service on Windows 7 64 bit. I want to:

1. see what heap size is currently configured and active
2. update the service configuration persistently to use a different heap size
3. verify that Tomcat is actually using the new heap size.
4. do all that from the command line.

For 1 and 3: I naively tried to use jconsole, but I don't find the process there because Tomcat is running as Local System Service. While I found out how to run jconsole as Local System Account, it seems that JMX is deactivated when Tomcat is installed as service.

So finding out the currently used memory sizes by JMX seems to be at least very complicated (possibly enabling JMX remote, which should be over TLS...).

For 2: I suppose this is the corresponding place of Tomcat's documentation, which reads:

 To update the service parameters, you need to use the //US// parameter.

 Update the service named 'Tomcat9'
 C:\> tomcat9 //US//Tomcat9 --Description="Apache Tomcat Server - http://tomcat.apache.org/ " ^
      --Startup=auto --Classpath=%JAVA_HOME%\lib\tools.jar;%CATALINA_HOME%\bin\bootstrap.jar

But I don't understand that text sufficiently to apply it to my problem. Especially, I don't want to change other parameters (like description, startup etc.).

As far as I understand, when run as a service, configuration is stored in the Windows registry, so the usual configuration in tomcat/conf does not or at least does only partly apply.

Please note that this question is not about installing Tomcat, but about modifying an existing installation. Also I am not interested in some hacky way to get the desired result (somehow), but in the best practice to do that; and it would be perfect to have links to reference documentation for that.