I have built a house of two floors.
And because I am a beginner, how can I from the security codes to choose a part from the floor namely when you press this point fall down
Since you’re a beginner, you should really check out the tutorials. They are great videos, and I promise you’ll learn much faster and easier than using the forums. Unity Learn
If you get stuck and something isn’t working the way you’d expect, that’s a good time to ask the community for help. It’s just really hard for us to help unless you’ve tried it yourself first.
I’ll second this just to reinforce that we’re not trying to be anti-helpful. We love helping (and being helped) but it’s almost pointless for us to try before the “helpee” has gone through all tutorials and truly believes they’ve exhausted other training resources. We want to help troubleshoot issues, not teach basic functionality, if that makes sense. Again, not trying to be anti-helpful. It just works best for you, and us. 
You’ll be much happier once you finish the tutorials and have a really solid understanding of how stuff works. It opens a lot of fun doors.
Ok thank you.
I have got a question. When I first open Unity, I saw a windows with a package.
What is that package in Asset?
If I’m understanding you correctly, that’s the list of optional modules you can add to any project. Things like base particle effects, terrain assets, player controllers, etc. If I were you I’d take some time to read over these guides to familiarize yourself with the core of Unity. http://docs.unity3d.com/Manual/UnityOverview.html