Here’s a script got from Y2mate.com I tweaked a bit, instead of downloading video it downloads Mp3.

When I click on the “Download” button. It opens this page , on this page you can see the default option is 128kbps.

Is it possible to change it? How can I change it to 320kbps as default?

Just re-Formatted same code for readability–

// ==UserScript==
// @name        Y2mate Converter Button
// @namespace   https://y2mate.com/
// @version     1.5
// @date        2019-07-23
// @author      A Koi
// @description Y2mate Downloader: Download Video and Audio for free
// @homepage    https://y2mate.com/
// @icon        https://y2mate.com/themes/images/logo.png
// @icon64      https://y2mate.com/themes/images/logo.png
// @updateURL   https://app.y2mate.com/extensions/chrome/helper.meta.js
// @downloadURL https://app.y2mate.com/extensions/chrome/helper.user.js
// @include     http://*
// @include     https://*
// @run-at      document-end
// @grant       GM_listValues
// @grant       GM_setValue
// @grant       GM_getValue
// @grant       GM_deleteValue
// @grant       GM_xmlhttpRequest
// @grant       GM_info
// @grant       GM_openInTab
// @grant       GM_setClipboard
// @grant       GM_registerMenuCommand
// @grant       GM_unregisterMenuCommand
// @grant       GM_notification
// @grant       GM_download
// @grant       GM.info
// @grant       GM.listValues
// @grant       GM.setValue
// @grant       GM.getValue
// @grant       GM.deleteValue
// @grant       GM.openInTab
// @grant       GM.setClipboard