I have old Toshiba z930 laptop and i wanted to install windows 10 on it. (original was windows 8)
Make a bootable ISO usb memory for this, i used Rufus to do that.
Download windows 10 ISO.
make your bootable usb flash disk using rufus.
reboot you computer.
when computer opens press f12 multiple times until it asks for you to choose boot options.
select usb and enter
computer will search for the boot disk and wait a while
then you should see the windows installation screen