[рони]

Artur_Hopf,

let button = document.querySelectorAll('div.modal-body div.list-group button');
let activeItem = button[0];
[].forEach.call( button, function( item )
{
    item.addEventListener( 'click', function()
    {
        activeItem.classList.remove('active');
        activeItem = item;
        activeItem.classList.add('active');
     });
});