There are several good answers on here but I wanted to add one since it may be helpful for users like me who have Tomcat installed as a service on a Windows machine.
Option 3 here: http://www.codejava.net/servers/tomcat/4-ways-to-change-jre-for-tomcat
Basically, open tomcatw.exe and point Tomcat to the version of the JVM you need to use then restart the service. Ensure your deployed applications still work as well.