I had the same issue and contacted Wolfram support. You can do this: On your current laptop you should evaluate DeleteFile[$PasswordFile], this will remove the existing activation information. Then when you start Mathematica again you will be asked to activate, now enter the Personal Use Activation Key.

When you install Mathematica on the new laptop you will need to complete the system transfer form at http://support.wolfram.com/12412, this will then allow you to activate Mathematica with the Activation Key for Lxxx-xxxxx (your key)

You can access your license information and downloads via your User Portal at user.wolfram.com.

Go to the "My Products and Services" page and select a product to see the license information and downloads.

If you have any additional questions, please do not hesitate to contact us.

[$PasswordFile]

Is it possible to delete it manually? Where can I find it?

So, after clearing all contents from all mentioned directories, and if I reinstalled windows 10 again, can I activate it regularly and it will be activated?

I dont know, since I have never had reason to reinstall Windows. Try it. Or ask Wolfram Support.