How to change state when Button pressed?

Your onButtonPress should be inside class since you want to do setState

export default class Hella extends Component {
  constructor(props) {
    ...
  }

  onButtonPress = () => {
    this.setState({
      text: this.state.mimin
    });
  }

  render() {
    return (
      ...
      <Button
        onPress={this.onButtonPress}
        ...
      />
      ...
    );
  }
}

React Native uses a lot of React concepts. Learning basics of React will help you a lot https://facebook.github.io/react/tutorial/tutorial.html