<div class="slider-wrapper">
<button data-step="-1">-</button>
<div class="slider"></div>
<button data-step="+1">+</button>
</div>
$('.slider').slider({ /* настройки слайдера */ });
$('.slider-wrapper [data-step]').button().click(function() {
const $slider = $(this).closest('.slider-wrapper').find('.ui-slider');
const step = +this.dataset.step;
const { min, max, value } = $slider.slider('option');
$slider.slider('option', 'value', Math.max(min, Math.min(max, +value + step)));
});
https://jsfiddle.net/142touep/