You can set the style properties of an existing element:

const node = document.querySelector('[title="40% Discount!"]')

node.style.color = "red";

More from here

In modern Webdevelopment you should not use .style in JS as it adds it as inline-style and as such has the highest specificity weight that only get overwritten by !important. The modern solution should be to add a class.

Use classList.add('class-name') to add a class to that element that contain color: red; instead!

var node = document.querySelector('[title="40% Discount!"]').classList.add('red');

.red {
  color: red;
}

<div title="40% Discount!">40% off</div>

I selected an element by title. From now on I want to update the css of this text. For ex. ‘color: red;’ how should I move on?

var node = document.querySelector('[title="40% Discount!"]');