index.js

class FirstComponent extends Component {
  state = {
    a: 2
  }

  changeState = val => {
    this.setState({
      a: val
    })
  }
  
  render() {
    return (
      <div>
        <SecondComponent function={this.changeState} />
      </div>
    )
  }
}

// onClick it will change state of first component
class SecondComponent extends Component {
  render() {
    return (
      <div>
        <button onClick={() => {
          this.props.function(3)
        }} />
      </div>
    )
  }
}