How to change the ROOT application?

There are three methods:

• First shutdown your Tomcat from the its bin directory (sh shutdown.sh). Then delete all the content of your Tomcat webapps folder (rm -fr *). Then rename your WAR file to ROOT.war, and finally start your Tomcat from the bin directory (sh startup.sh).

• Leave your war file in $CATALINA_BASE/webapps under its original name. Turn off
  autoDeploy and deployOnStartup in your Host element in the server.xml file.
  Explicitly define all application Contexts in server.xml, specifying both the path
  and docBase attributes. You must do this because you have disabled all the Tomcat
  auto-deploy mechanisms, and Tomcat will not deploy your applications anymore
  unless it finds their Context in the server.xml.

  second method: in order to make any change to any
  application, you will have to stop and restart Tomcat.

• Place your WAR file outside of $CATALINA_BASE/webapps (it must be outside
  to prevent double deployment). Place a context file named ROOT.xml in $CATALINA_BASE/conf/. The single element in this context file MUST have a
  docBase attribute pointing to the location of your WAR file. The path element
  should not be set – it is derived from the name of the .xml file, in this
  case ROOT.xml. See the documentation for the Context container for details.

Reference