Slick Slider: How to set arrows inside and not outside?

I think you can simply change the CSS style.

.slick-prev {
    left: 100px;
    background: red; /*to notice it, is white*/
}
.slick-next {
    right: 100px;
    background: red; /*to notice it, is white*/
}

add z-index: 1 to the answer by UberKaeL to force prev-arrow display on top of image

.slick-prev {
  left: 100px;
  background: red; /*to notice it, is white*/
  z-index: 1;
}
.slick-next {
  right: 100px;
  background: red; /*to notice it, is white*/
}