This option in the Windows 10 will work only with with Threshold 2 update.

  1. Download the latest build of windows 10.
  2. Boot to disk and start installation, when reaching the windows 10 key, skip the part where you need to enter the Windows 10 key.
  3. Finish the installation so that Windows 10 copy will be installed as an evaluation copy.
  4. Go to Settings > Update & security > Activation and selecting Change Product Key
  5. Enter the product key you have from your former windows 7 or windows 8 key and activate the product