If you’ve installed jupyterlab from source, and changed something in packages/, you can build and run the changes by doing jupyter lab --dev-mode. This runs JLab from the dev_mode folder, where your changes are applied. A red line will appear at the top of your screen, indicating you are running from source.
It depends on what you want to do. If you want to make some changes to the core extensions, you’ll need to build your own distribution of JLab with your extensions replacing the built-in extensions. For example, you can make a new plugin from the core one and install the new one and disable the core one. Or you can build a new distribution where you pick and choose between your packages and the core packages - see the app example at https://github.com/jupyterlab/jupyterlab/tree/master/examples/app for something like this.
In general, we try to make the core flexible enough that you can make whatever customizations you need in a new extension. What exactly are you trying to do that you need to modify core extensions for?