I am using Bootstrap 4 Beta 2 version to do a carousel. The code looks like below:
You just need to change the icon path in case you have one, using the inspect element you can get the class name. I used Mozilla to get the class name and then I used two svg icons for changing the default icons to custom icons.
.carousel-control-prev-icon {
background-image: url("images/carousel/back.svg");
}
.carousel-control-next-icon {
background-image: url("images/carousel/next.svg");
}