To resolve the issue of an app crashing instantly upon launch on your iPhone, follow these steps:

1. Check for Updates
Ensure that both the app and your iOS are updated to the latest versions. Open the App Store, search for the app, and look for any available updates. Also, go to Settings > General > Software Update to check for iOS updates.

2. Restart Your iPhone
A simple restart can often fix temporary glitches. To do this, press and hold the power button until you see the slider to turn off your device. Slide it to power off; after a minute, turn the device back on.

3. Clear App Data
If the app allows, clear its cache or data. You can do this by uninstalling and reinstalling the app, which clears any corrupted data that may be causing the crash.

4. Check for Crash Logs
If you have access to crash logs (especially useful if you are a developer), you can review them to pinpoint the issue. For TestFlight apps, crash logs can provide detailed error messages that can guide further troubleshooting - learn more.

5. Reset Settings
If the crashes persist, consider resetting all settings on your iPhone. This does not delete your data but resets system settings. Go to Settings > General > Transfer or Reset iPhone > Reset > Reset All Settings.

6. Reinstall the App
Uninstall the app by pressing and holding its icon and selecting 'Remove App.' Then, reinstall it from the App Store. This can solve issues caused by corrupted app data.

7. Contact App Support
If the issue continues after trying the steps above, contact the app's customer support for additional help. They may be aware of the issue and could provide specific troubleshooting steps.

8. Check Device Storage
Make sure that your device has enough storage space. If your storage is almost full, it could lead to performance issues with apps. Check this by going to Settings > General > iPhone Storage.

By systematically following these steps, you should be able to troubleshoot and resolve the app crashing issue on your iPhone.