Tapestry 5.6.4 is a drop-in replacement and recommended upgrade for Tapestry 5.4, 5.5 and 5.6 releases. To upgrade, just update the dependency in your build configuration (Maven POM, Gradle build script, etc.) – or Download the new JAR file -- and the new version will just work. However, please review the How to Upgrade instructions before upgrading.
Please check all the details in Release Notes 5.6.4.