Windows 10 not activated after uninstalling product key

Here's How:

1. Open an elevated command prompt.

2. Copy and paste the command below into the elevated command prompt, and press Enter to uninstall the product key. (see screenshot below)

slmgr /upk

3. Click/tap on OK when the product key has successfully been uninstalled. (see screenshot below)

4. Copy and paste the command below into the elevated command prompt, and press Enter. (see screenshot below)

Note
This will clear the previously installed product key from the registry so that it can no longer be viewed by product key finder programs like ShowKeyPlus.

This will not remove an OEM product key embedded in the UEFI firmware chip.

 slmgr /cpky

5. Click/tap on OK when the product key has successfully been cleared from the registry. (see screenshot below)

Source : http://www.tenforums.com/tutorials/35979-product-key-uninstall-deactivate-windows-10-a.html