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Hello and thank you for your question. I look forward to assisting you today. If the new computer came with Windows 10 then the only way you can change back will be to purchase a Windows 8 disc and install it.
You could also remove your hard drive from your old computer and with the proper adapters, make it a portable USB drive and copy any remaining files you have on it to the new computer.
What type of hard drive is it?
Trust me, this is as easy as plugging in the adapter to the drive, then plugging it into one of your USB ports on your computer.
Did it come from a laptop?
Do you know the make and model?
Okay, go to this LINK. This is the adapter you will need to connect your drive to your new computer.
Did you have a chance to check out the adapter?