I deleted my texting app on my phone. How can I get it back?

running iOS 14?

From your Home Screen, swipe left until you get to the App Library.

Swipe downward and you'll get an alphabetical list of your apps.

Locate the app in the list or type the app name into the search field

Once you've located the app, tap/hold until it raises/selects, then drag it and it'll pop out to your Home Screen

Organize the Home Screen and App Library on your iPhone - Apple Support

The Messages app is not one of the apps that can be deleted but with iOS 14 it can be removed from the home screen. If you have iOS 14, go to the App Library (swipe left past your last home screen). From there you can touch and hold an app and slide it left back to the home screen.

If that is not the cause, try using Search to see if that will locate it: https://support.apple.com/en-us/HT201285