You won't be able to keep your current Windows 8 installation when running a basic upgrade to Windows 10. On the other hand, you could download the Windows 10 ISO using the upgrade software and try to install it beside you current Windows 8 (it requires to create a disk partition) but I do not recommend doing such a thing.
Instead, just give a try to Windows 10 and if you want to go back to Windows 8, there is way (Note that this option is only available for a month after you upgrade to Windows 10).
And don't worry, an upgrade will keep your files/programs in place.
Please note that Windows 10 upgrade is no longer available for free. You'll need to pay for it.