I bought my laptop brand new from a authorized store and got windows 8.1 pre-installed. Then I updated to windows 10, I used it for more than a month and it was working fine but now it starts giving problems like my laptop slows down, takes too much time to start up. So due to this reason I want to switch back to windows 8.1 which had come pre-installed. But the issue was my 'windows.old' file is been deleted and I have also passed the one month time period to switch back. So I went to the Microsoft download page and downloaded the windows 8.1 iso file. But during the time of installation it asks me for a product key, to which sometimes the product key is present under the laptop with a Microsoft sticker. But my laptop didn't have that, now is there a way to get my windows 8.1 product key?
Microsoft community please help if possible.