HiDood :
i need to reinstall win8 because i installed kmspico... so how about upgrading to windows 10?
but how to upgrade without wiping the hard drive data?
You save your current critical data elsewhere first.
Then you install a new OS, either Win 8.1 or the Win 10 Tech Preview.
If you install the Win 10, you will have to actually buy the OS sometimes between release date (July/August?) and Oct 1.
But you seem to be asking to install the new OS without disturbing your existing data.
That is a 'maybe'. But why risk it? Have your critical data elsewhere first.