Some of the users didn’t receive the Window 10 upgrade alert (GWX) on their taskbar. Therefore not able to upgrade to Windows 10.
There is another way to upgrade your Windows:
1) Go to http://www.microsoft.com/en-us/software-download/windows10
2) Click on Download Tool Now (64-bit version) – for 64bit windows. You can check it by going to Control Panel, System. There will show you your Windows edition and System Type.
3) Run MediaCreationToolx64.exe
4) Select Upgrade this PC now and click Next
5) Follow on-screen to complete your Windows upgrade
6) After Windows upgrade, remember to save a copy of your computer info (which includes Windows Product key) using Belarc Advisor (http://www.belarc.com/free_download.html).