How to change HTML Attribude with JavaScript

I have a div where is an HTML Attribute called data-theme inside. I want to ask how do I change the value of the Attribute called data-theme. For example: From: data-theme="dark" to data-theme="light". The value is stored in a variable as a string.

My div:

<div id="myId" data-theme="dark"></div>

I want to use normal JavaScript.

>Solution :

document.getElementById("myId").setAttribute("data-theme", "newValue")