When nip2 starts up it loads all of the definition files (files with a .def extension) it can find in the directories listed in your start path. You can change the start path in Preferences. By default, the start path lists just two areas: a personal start directory that nip2 makes in your home area, and the main system nip2 start directory containing all the standard toolkits.
If there are two files with the same name on the start path, then nip2 will only load the first one. This means that if you modify one of nip2’s built-in menus and save it to your personal start directory, in future you’ll just see your personalised version.
You can load or reload a toolkit at any time with the File / Open Toolkit menu item in the program window. If you open a toolkit with the same name as an existing toolkit, nip2 will remove the old toolkit before it loads the new one.