How to change the app name in react-native(in android and IOS):

In Android:

To change the app name in react native first go to your project folder and open this path in VScode,
android/app/src/main/res/values/string.xml
Now you can easily change the app name in the string tag.
For example:
<string name="app_name">YOUR_APP_NAME</string>

After changing the app name, uninstall the previous app from your device and run these commands:
$ cd android
$ ./gradlew clean
$ cd ..
$ react-native run-android

For IOS:

Open info.plist, change the value after the key to your app name. Eg.
<key>CFBundleDisplayName</key>
<string>My New App Name</string>
Now uninstall the previous app from your device. Run
$ npm install
$ pod install
Now simply, you can install the new app on your device.
Or you can also change the app name from Xcode.
Go to Xcode, open your project in Xcode, and find the targets tab. Select your project and change the hostname with your app name.