1. Download the desired Tomcat version from its official site. You can click here to get Tomcat 8. 
2. Make sure that you pick the bundle meant for Windows as highlighted below.
   
3. Extract the content and put it into the Joget's installation directory. At the time of writing, "apache-tomcat-8.0.33" is the version downloaded.
   
4. Copy the jw.war and jwdesigner.war from existing tomcat's webapps folder into the new one (i.e. from \apache-tomcat-8.0.20\webapp\ to \apache-tomcat-8.0.33\webapp\)

5. Modify joget.start to point to the new tomcat folder.

   set CATALINA_HOME=.\apache-tomcat-8.0.20

   set CATALINA_HOME=.\apache-tomcat-8.0.33

6. Carry on at Installing Joget as a Windows Service.

After you have successfully added Tomcat as a service, you may also want to add MySQL as a service and make Tomcat dependent on MySQL as well.

1. Installing MySQL as a Windows Service
2. Adding MySQL service as a Dependency to Tomcat service

Reference: Apache Tomcat 8 - Windows service HOW-TO