Java 6 (or "1.6") was released more than 2 years ago: http://en.wikipedia.org/wiki/Java_version_history. Whether you can update depends on whether there's a JDK 6 for your operating system version available. For most common desktop OS versions upgrading is no problem.
Well, in relation to java 1.5 and 1.6 there are not too much differences. But to certification i recommend for you take the version 1.6.
This test have more question and the % to pass, is more high than 1.5. But is a good certification.