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1, Do not upgrade tomcat 8.5 over version 7 - install separate version and make sure you start the server after installing
2. You will need tomcat 8.5 installed before you upgrade
Start the service
configure all the settings as you have v7 configured
Also look at the following for more detail
Recommend that you do all this first on your test server so you are familiar with what to do before attempting on production.