Buy a copy of Windows 8.1 and perform a clean install. Be aware you will lose all the program installed on the laptop and any personal files. You might want to check with the manufacturer for Windows 8.1 drivers and have those ready on a USB flash drive. Especially important are the chipset and networking, Ethernet and Wi-Fi, so you can get it back up and running quickly.
Here's a guide for when you're ready to start.
i dont see why you would want to. fact 1) the hardware may be so new it wont work on anything less than 10. fact 2) since 10 has only just come out, it should get much more reliable SOON(we all hope). fact 3) since it came with 10 it shouldnt really have any issues with 10. if you want other options, shrink the drive a bit, add a new partition, and install VirtualBox that allows you to install and run any OS in that space, like starting an APP in 10. you can always delete the partition later and stretch the original partition back to its original size. whatever you choose, MAKE YOUR RECOVERY MEDIA NOW so ifyou screw it all up, you can at least confidently know you can get it all back. ** note some cpu's are so cut down, they may not have enough features to make use of VirtualBox.
You can't without a clean install.
Get a Win8 iso and put it on a CD/DVD/USB
Check out a tutorial on how to clean install windows
If you're scared of doing it because you might break something:
don't be, installing windows is easier than changing a diaper
If you're still unsure of how to do it, you can take it to a service, but it will most probably cost you
You would need to buy Win 8 and install it over your Win 10
Get an installation CD of Win8 then install from that.