JSFiddle - React, Tailwind, and code Playground

HTML

<p id="id">
    Some mixed \u6F22\u5B57 text
    <strong>Nothing to replace here</strong>
</p>
<p><button onclick="$('id').decodeUnicode();">Decode</button></p>

JavaScript

Element.addMethods({
    // element is Prototype-extended HTMLElement
    // nodeType is a Node.* constant
    // callback is a function where first argument is a Node
    forEachDescendant: function (element, nodeType, callback)
    {
        element = $(element);
        if (!element) return;
        var node = element.firstChild;
        while (node) {
            if (node.nodeType == nodeType) {
                callback(node);
            }

            if(node.hasChildNodes()) {
                node = node.firstChild;
            }
            else {
                while(node.nextSibling == null && node.parentNode != element) {
                    node = node.parentNode;
                }
                node = node.nextSibling;
            }
        }
    },
    decodeUnicode: function (element)
    {
        var regex = /\\u([0-9A-Z]{4,6})/g;
        Element.forEachDescendant(element, Node.TEXT_NODE, function(node) {
            // regex.test fails faster than regex.exec for non-matching nodes
            if (regex.test(node.data)) {
                // only update when necessary
                node.data = node.data.replace(regex, function(_, code) {
                    // code is hexidecimal captured from regex
                    return String.fromCharCode(parseInt(code, 16));
                });
            }
        });
    }
});