How do I get weather icon back on my iPhone after it was accidentally deleted?

Hello,

Restore a built-in app that you removed

1. Go to the App Store.
2. Search for the app. Make sure that you use the exact name of the app. Find the correct name of built-in apps.
3. Tap to restore the app.
4. Wait for the app to restore, then open it from your Home screen.

From:

Remove built-in Apple apps from the Home screen on your iOS device with iOS 10 and later - Apple Support