JSFiddle - React, Tailwind, and code Playground

HTML

<input type="text" id="txtDesconto" value="0">
<button id="aumentaDesconto">+</button>

JavaScript

var input_desconto = document.getElementById("txtDesconto"),
    aumenta_desconto = document.getElementById("aumentaDesconto");

aumentaDesconto.addEventListener("click", function () {
    input_desconto.value = parseInt(input_desconto.value, 10) + 1;
}, false);