Trying to upgrade to Windows 10

Ok needing some help here's whats going on. I have a windows 8.1 bing laptop I'm trying to upgrade to windows 10 for free which I can still do windows support said its still possible since my windows 8.1 bing was already active. So here's the problem. My laptop is a hp stream 11 with built in hard drive so there's no way to just reemove and upgrade the size. So the hard drive is currently a 21. something gb hard drive and there's only like 8 gb free.

So the windows installer says there's not enough free space and since I just reset the pc yesterday to remove previous owners user name and password. I don't know what could be taking up the space. I do have a 16gb flash drive with windows 10 pro upgrade onto it from when I upgraded my desktop pc.

Here's the crazy part since my hard drive in the laptop is a complete 21. something gb hard drive is it possible to format it and remove the current windows from it and install the windows 10 pro from the flash drive onto the laptop. I know it might sound odd the way I'm explaining this but I'm new to laptops. And if I could install from the usb flash drive how would I go by doing it if I don't know the product key. Is it possible to find it on my desktop and also use the same one on my laptop if I can upgrade it like that.

I hope I'm commenting in the correct area.