Change method

App.js

import React from 'react';
import { StyleSheet, Text, View, Button, TextInput } from 'react-native';

export default class App extends React.Component {

  constructor(props) {
    super(props);
  }

  render() {
    return (
      <View style={styles.container}>
        <Text>Open up App.js to start working on your app!</Text>
        <Text>Changes you make will automatically reload.</Text>
        <Text>Shake your phone to open the developer menu.</Text>
        <Button title='This is my button' onPress={this.handlerPress}/>
        <TextInput onChangeText={this.props.changeText}/>
      </View>
    );
  }

  handlerPress = () => {
    this.props.myMethod();
  }

}

const styles = StyleSheet.create({
  container: {
    flex: 1,
    backgroundColor: '#fff',
    alignItems: 'center',
    justifyContent: 'center',
  },
});