For reasons only known to Microsoft since the AU update you can no longer disable it from the desktop. However you can make the following registry change to work round this.
Press Windows+R and run “regedit” and navigate to the following key:
If you don’t see a “Windows Search” below the Windows folder, right-click the Windows folder and select New > Key and name it “Windows Search”. Right-click the “Windows Search” key (folder) and select New > DWORD (32-bit) Value. Name the value “AllowCortana”. This should automatically be set the value to 0. If not set it to 0. A zero value disables Cortana. You can now close the registry editor. The change may immediately take effect. If not sign out and back in or restart your computer to implement the change.
Note a (default) subkey value 0 is automatically created under “Windows Search”. Don’t worry about this.
To enable Cortana set “AllowCortana” to 1.
For more details click on the following link: