If you delete the NowPatient Diabetes app, you will lose all data recorded in the app. NowPatient Diabetes does not keep a restorable copy of your data. All your data required to use the app is stored on your iPhone. When you delete the app, Apple removes all data stored on the phone that is associated with that app.
You can reinstall the NowPatient Diabetes app through the Apple App Store at any time and start again.
If you accidentally delete the NowPatient Diabetes app and use iCloud to backup your phone data, you can restore a backup version of your entire phone. Instructions from Apple on how to do this are here.