As you're no doubt aware, today is Windows 10 launch day. If you're already registered for the new OS, now it's just a waiting game as Microsoft pushes out the update.
Don't want to wait? You don't have to: There's actually a Windows 10 download page where you can grab the OS right now. Although it appears to be focused on creating installation media (e.g., a DVD or flash drive), you can use it for in-place upgrades -- no external media required.
Needless to say, you should definitely back up your data first. You may also want to consider installing a rollback tool like Easeus System GoBack. Although Windows 10 has a rollback feature of its own, the option expires after one month. GoBack should work indefinitely.
Update 7/31/15: I tested System GoBack on two different laptops, and both times the result was the same: a generic error message when trying to "go back" to Windows 8.1. Your mileage may vary, of course, but at this point it's hard to recommend the tool.
Next, head to the Windows 10 download page, scroll down a bit and click Download Tool Now. (Most users will want the 64-bit version, but obviously you should go 32-bit if that's the version of Windows your system already runs.)
Run the tool, then choose "Upgrade this PC." From there it's just a matter of waiting while Windows 10 downloads, verifies and installs -- with the inevitable reboot or two along the way.
I just completed this process on a Nextbook Flexx 11, one that had already been registered for the Windows 10 upgrade. It took the better part of an hour. Your mileage may vary, of course, and although everything worked fine at my end, the smarter move is probably to wait until the "official" update arrives on your PC.
But if you just can't wait, hit the comments and let us know how your "upgrade-by-force" fared.