Hi stucki,
That is actually a great question. I haven’t done this myself yet, but from what I have heard from other users who have updated, they simply “Add to Project” in a new project and copy/replace the nature pack files in their actual working project. I am not sure if this is the right way to do it, but I have heard it working for people. You can always ask by creating a thread to see how best to update packs. You can also email EPIC to see what they suggest as the best way to update your project with the new files at this email.
marketplace-support@unrealengine.com
I hope that helps and let me know if you experience any issues with the update! Thanks!