I am using drools older version 5.4.0 in my project by using this version I got one issue which we can not pass more than 255 arguments but this bug is resolved in drools latest version 5.5.0 and 6.0.0.
So Now:
Assuming you are using a Drools library in your projects, you should create a new runtime with the version of Drools you want to use (download the bin artefact, unzip and create a runtime based on that folder), and use that for your project. If you are using maven to define your dependencies, simply update the version of Drools you want to use in your pom.xml.
Upgrading from 5.4 to 5.5 shouldn't really impact working code. Upgrading to 6.0 should also be backwards compatible if you have the right jars in your classpath, but might be slightly more tricky (as that is a mayor upgrade).
You can also update your Eclipse plugin (for example using an update site like the JBossTools update site for Eclipse Kepler: http://download.jboss.org/jbosstools/updates/integration/kepler/integration-stack/aggregate/ ) but I don't think that is absolutely necessary.
Kris