How do I create a patch for a preexisting NetBeans module?

If you need to patch an existing module, you can place a JAR file relative to the original. For example, to patch ide5/modules/org-openide-example.jar you make a JAR like ide5/modules/patches/org-openide-example/mypatch.jar. The mypatch part of your JAR file patch can be named anything you like. The JAR file should only contain those classes you want to patch. It does not need a manifest, though an empty manifest is harmless.

The patch must be in the same cluster as the original. (Issue 69794) If you want to create an NBM containing a patch, you must ensure it will be installed in the same cluster (use the nbm.target.cluster property), but note that you cannot test such a dummy module as part of a module suite (since this property is interpreted only by Plugin Manager). If you are distributing a complete application including a patch to the NB Platform, you will need to either manually preinstall the patch JAR in your copy of the Platform; or override your build-zip target to include the JAR in the final ZIP (in which case testing using Run Project will not have the patch active).