Here are the steps to change your display from mirroring or extending on Windows.
For Mac click on the Duet icon in the menu bar.
You should see a bar displaying an image of an iPad. Hover over this bar.
Doing this will open up the display menu for your iOS device. You will most likely see that the only option under the "Mirror Displays" section is enabled. To stop mirroring, please choose any of the four options under the "Extend Displays" section.
If the menu doesn't appear, then please click on the Gear icon and terminate the Mac app by clicking on "Quit". This will close Duet. Relaunch the Duet Mac app. If you have your iPad still connected with Duet running, it should automatically reconnect once the Mac app relaunches.
Then hover your mouse cursor over the section that reads "iPad, Mirrored for X Minutes". The options should now appear.