Undeploy the 8.5.3 .war file from your application server.
Important
Do not just copy the 9.0.war file over the 8.5.3 .war file in your application server directory. Genesys recommends undeploying the previous file first and letting the application server clean up its files, then deploying the new .war file.
Ensure you are running the latest supported Tomcat 8.5 application server (Note: Tomcat 9 is not yet supported).
Ensure your application server uses the latest Oracle Java 8 or Open JDK 8.