The reason they don't say how to do it is because JasperReports isn't a standalone app that end users upgrade themselves - it's a library used for reporting features. If you didn't build the application that uses JasperReports yourself you don't really have any reason to upgrade to 4.0.2.
If you did build the application yourself, then you just need to swap the jars from the old jasperreports out with the new ones and then test to make sure there weren't any breaking changes made to the API. (The changelog doesn't seem to indicate there were any, but you don't ever indicate what version you are upgrading from so perhaps there are some)
If you are using something like maven to build whatever you are using, you can update the version in the dependency information and run a new build and after all your existing unit tests pass you can run your app with the new version of jasperreports and make sure it is all still working.