The license for StaffPad is held and controlled by the App Stores on each platform. Therefore, you'll need to go back to the App Store to redownload StaffPad on your new device.
Please make sure you update to the latest versions of Windows 10 and iOS first; StaffPad does require the newest operating system versions on both platforms.
You can re-install the app by opening the App Store, tapping your Account photo in the top right, and choosing "Purchased". This will show you a list of all the purchased apps and the content linked to your Apple ID. Simply download StaffPad by pressing on the download icon in the list.
Please refer to this article for more information: https://support.apple.com/sl-si/HT201272
If you have a new device, have reset Windows or have previously uninstalled StaffPad in the past, you may be wondering how to get StaffPad installed again.
Since StaffPad is an app that you purchase and download through the Microsoft Store app, you can always re-install from the Store at any time, for example if you've previously uninstalled the app, or have purchased a new device.
StaffPad is linked to your Microsoft Account within the Store, so make sure you've signed into the Microsoft Store app with the same account credentials as you used when you first purchased the app.
Once you've signed in, search the Store again for StaffPad and choose "Install".
You can also view a list of all your previously purchased apps, games, music, movies etc from the Store by clicking the account icon and selecting "My Library". From there, you can re-download anything you've previously purchased from the Store, including StaffPad.