// U contructoru dodaj ovo
this.prev = this.prev.bind(this);
this.next = this.next.bind(this);

// Unutar classe negdje prije rendera dodaj ove funkcije
 prev(e){
    let cl = this.state.classList.map(item=>'');
    let num = (this.state.slides.length+this.state.current-1)%this.state.slides.length;
    cl[num] = 'in'
    this.setState({current:num,currentTime:0,classList:cl,dotCounter:num})
  }
  next(e){
    let cl = this.state.classList.map(item=>'');
    let num = (this.state.slides.length+this.state.current+1)%this.state.slides.length;
    cl[num] = 'in'
    this.setState({current:num,currentTime:0,classList:cl,dotCounter:num})
  }

// U RENDER NA DNU DODAJ OVO
<div className="prev" onClick={this.prev}/>
<div className="next" onClick={this.next}/>

// U CSS DODAJ OVO I U public/img dodaj slike za left i right png koje hoces i eventuano malo sredi css
.prev{
  position: absolute;
  top: 50%;
  left: 20px;
  width: 75px;
  height: 75px;
  background-image: url('/img/left.png');
  background-size: 100% 100%;
  cursor: pointer;
}
.next{
  position: absolute;
  top: 50%;
  right: 20px;
  width: 100px;
  height: 100px;
  background-image: url('/img/right.png');
  background-size: 100% 100%;
  cursor: pointer;
}