Mac
If you're using a Mac, open System Preferences > Displays > Arrangement – from there you can click and drag the screens to position them as you wish. If for some reason you're not seeing the "Arrangement" tab, your iPad is most likely not being recognized by your Mac – check your cable and/or try connecting to a different port.
PC
If you’re using a PC (Windows 10), please go to Settings > System > Display and click and drag screens to your desired preference.