How can I realize a sildeshow which can be controlled by two buttons

I don’t know how to control my sildeshow by buttons, I use timer to make it come true now

    var index1 = 0, len1, timer;
    var hisBox = document.getElementsByClassName("history");

    var count = document.getElementsByClassName("left-arrow");
    len1 = hisBox.length;
    hisBox[index1].style.display = 'block';
    for(var i=1; i<len1; i++) {
        hisBox[i].style.display = 'none';
    }

    function hisSlideShow() {
        index1 ++;
       
        if(index1 >= len1) index1 = 0;
        for(var i=0; i<len1; i++) {
            hisBox[i].style.display = 'none';
        }

        hisBox[index1].style.display = 'block';
    }

    timer = setInterval(hisSlideShow, 5000);