As you've had Win 10 installed previously then the key will be no issue.
This is how I'd go about it. I would first create an image of the whole disk to an external USB drive using Macrium free, Link , and create its recovery medium (USB or DVD) then at least you have that to fall back on. Plenty of info on the net how to but just ask here if needed.
Once done I would back up all your DATA to the external drive as well as I would recommend doing a clean install.
Download Windows 10 from here - selecting "Using the tool to create installation media (USB flash drive, DVD, or ISO file) to install Windows 10 on a different PC". Check your system type - 64/32 bit. Once created - boot from the Win 10 media and when it is on the 'select where to install screen' use the option to delete partitions so you end up with just one unallocated space. Select that to install Windows 10. During the install, when it asks for the key, tick the box that you haven't got one and the installation will continue.