Note that the Windows 10 upgrade ("upgrade") process is:
Thus, I recommend performing a full system backup using something like CloneZilla before following these instructions. Sorry the CSS is still bad, but all of this is a link to a tutorial. Link ends here. You'll need a device bigger than your computer's hard drive (like an external HDD) to save the backup to, but given that you're still using WIndows 7 your computer's hard drive is probably small enough that you'll be able to buy one.