if you have automatic updates enabled, I'm surprised that it hasn't already.
If you don't want it to, you can install and run GWX Control Panel.
http://ultimateoutsider.com/downloads/
Without GWX Control Panel, it will update regardless of whether you select the option to do so, if Automatic Updates are enabled.
As Togo says, Windows 10 is an improvement on Windows 8.1 in most respects, but if you have any unusual hardware, you may experience problems (I had to roll back to 8.1 when my recording interface ceased working under Windows 10).
It's far far better, the only issue that I have had being an annoying low capacity on D drive - because of my Bullguard security....easiest cure to that is when it happens is to click on the action centre and then turn off the notification.
Im sure my pc has automatic updates enabled as i have seen other updates come up but still not the Windows 10 update yet.
Maybe i should do the update from the windows site myself then
if you are going to do it do a custom update so you can choose exactly what you want - you can force it by emptying your windows/softwaredistribution/downloads folder and then open a command prompt by right clicking on it and selecting as an administrator then typing wuauclt.exe/updatenow.
PLEASE make sure you backup all your personal stuff first including favourites / bookmarks.