how can i install this extension in slicer3D?
Model Undercuts Removal by Label Map Addition
Hi Luca, welcome to the Slicer community. You can find instructions for installing extensions here: https://www.slicer.org/wiki/Documentation/Nightly/SlicerApplication/ExtensionsManager#Installing_an_extension
Note that since Slicer is open-source, with all documentation available online, indexed by search engines, you should be able to find lots of answers by typing your question into Google.