The first most important thing is to not delete the app. If you do this without having set up sync with Dropbox or iCloud within Due > Settings > Sync before, you will lose all your data in Due.
You can attempt to resolve this issue by following this guide to Offload and Reinstall Due.
- Go to iOS Settings > General > iPhone Storage.
- You will see all of your apps listed, sorted by how much space each app takes.
- Tap on Due and then select Offload App.
This allows you to remove the app from your iPhone, while retaining all of your documents and data associated with the app. When you next tap on Due icon on your home screen, it will reinstall Due from the App Store.