Hi
http://www.microsoft.com/en-gb/software-download/windows10
(though this is UK page it may divert to another one for non UK connections )
choose 32 or 64 bit version according to Windows operating system you will do download from not version you intend to download.
When downloading Win 10 Choose the correct language, and version Home or Pro and 32 or 64 bit version
(no point in having 64 bit version if you have less than 4GB of RAM)
Go to website of PC or motherboard manufacturer to get windows 10 drivers
motherboard chipset, sound, ethernet network/wireless network , (power management is important for laptops)
graphics chipset may come from manufacturer of graphics chipset (AMD, NVidia or Intel)
If Windows 10 drivers not available maybe it is best to go back to Win 8 until the Win 10 drivers are available
Since you went to Windows 10 once before, the Windows Rediness (Get Windows 10 icon) should have reported any incompatibilities and Windows 10 may have all the drivers you need
Regarding data backup what do you do currently?
You will need a external USB hard drive, network connection to another PC or Cloud Backup service
With only small amounts of data a large good quality USB memory stick or set of DVD-R (or DVD-rw) disks may be enough
regards
Mike Barnes