[рони]

document.querySelector('.header__btn-left').addEventListener('click', sliderLeft);
let headerBg = document.querySelectorAll('.header__bg');
let idx = 0;
function sliderLeft(){
    headerBg[idx].classList.remove('header__bg_show');
    ++idx;
    if(idx == headerBg.length) idx = 0;
    headerBg[idx].classList.add('header__bg_show');
 }