Windows 10 technical preview update to windows 10

I do not want to be out of line, nor post something already posted, but I'm at a loss here. Downloaded win10 tech preview, and now it is expired. Build 9926. I do know that it is outdated, but did not get back to it in time.\
Now, it is leaving me a warning that my computer will shutdown every 2-3 hours, and eventually not restart at all until I install a new operating system.
This seems quite harsh on Microsoft's part, but I did not keep up with it. This is on an Acer netbook, previous 8.1, maybe two years old. I have been on the horn with Microsoft 4 times, and they tell me I will have to purchase (?) another copy of 8.1 in order to get Windows 10. I must be missing something, but I don't know where to turn.
The first technician got into my computer remotely, verified its authenticity, and went to a website to download win 10. It seemed to go well for a couple of hours, and then the comp. shut down! When it came back up, nothing.
Can anyone help me with this? I can't afford a new Operating system.

Can you download the Windows 8.1 ISO, burn a USB or DVD and use that to install windows 8.1 back on it?

Create installation media for Windows 8.1 - Windows Help

I can, but I won't have a code. I also have an 8.1 disk and an external DVD. The code would still be a problem though.

bigjohn262 said:

I can, but I won't have a code. I also have an 8.1 disk and an external DVD. The code would still be a problem though.

If the computer came with Windows 8 or 8.1 installed, then the product key will be stored in bios, you just install Windows 8.1 and it reads the product key from bios.

I will do that and let you know. Thank you from a Disabled Navy veteran. For your help, and your service.

If you get/have Windows running, you can run showkey from this forum and it will tell you what product keys you have on your system:
Showkey - Windows 10 Forums