How to customize 'Previous' and 'Next' buttons in 'Slick' slider

I created a carousel using 'Slick' but I want to change the 'Previous' and 'Next' buttons into arrows. How do I do that?

This is my code:

$('.horizontal').slick({
  slidesToShow: 2,
  slidesToScroll: 1,
  autoplay: true,
  autoplaySpeed: 100,
});

#slider img {
  width: 350px;
  height: 350px;
  margin-right: 100px;
}

<section id="slider" style="background-color:#fefefe;height: 800px;">
  <div class="horizontal">
    <img src="images/imags/1.jpg" class="before_after">
    <img src="images/imags/2.jpg" class="before_after">
    <img src="images/imags/3.jpg" class="before_after">
    <img src="images/imags/4.jpg" class="before_after">
    <img src="images/imags/5.jpg" class="before_after">
    <img src="images/imags/6.jpg" class="before_after">
    <img src="images/imags/8.jpg" class="before_after">
    <img src="images/imags/9.jpg" class="before_after">
    <img src="images/imags/10.jpg" class="before_after">
    <img src="images/imags/11.jpg" class="before_after">
    <img src="images/imags/12.jpg" class="before_after">
    <img src="images/imags/13.jpg" class="before_after">
    <img src="images/imags/14.jpg" class="before_after">
    <img src="images/imags/15.jpg" class="before_after">
    <img src="images/imags/16.jpg" class="before_after">
  </div>
</section>

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