reverse engineer build js file

I have a site ive been asked to work on, the code base ive been giving is missing the original build files and have been told they are no longer available.

I find it difficult to work of these built files as the js is built and not easy to read see bellow for sample

    var r = {};

function o(e) {
    if (r[e]) return r[e].exports;
    var n = r[e] = {i: e, l: !1, exports: {}};
    return t[e].call(n.exports, n, n.exports, o), n.l = !0, n.exports
}

o.m = t, o.c = r, o.d = function (e, n, t) {
    o.o(e, n) || Object.defineProperty(e, n, {enumerable: !0, get: t})
}, o.r = function (e) {
    "undefined" != typeof Symbol && Symbol.toStringTag && Object.defineProperty(e, Symbol.toStringTag, {value: "Module"}), Object.defineProperty(e, "__esModule", {value: !0})
}, o.t = function (n, e) {
    if (1 & e && (n = o(n)), 8 & e) return n;
    if (4 & e && "object" == typeof n && n && n.__esModule) return n;
    var t = Object.create(null);
    if (o.r(t), Object.defineProperty(t, "default", {
        enumerable: !0,
        value: n
    }), 2 & e && "string" != typeof n) for (var r in n) o.d(t, r, function (e) {
        return n[e]
    }.bind(null, r));
    return t
}, o.n = function (e) {
    var n = e && e.__esModule ? function () {
        return e.default
    } : function () {
        return e
    };
    return o.d(n, "a", n), n
}, o.o = function (e, n) {
    return Object.prototype.hasOwnProperty.call(e, n)
}, o.p = "", o(o.s = 4)

My question

Is there a way of turning this back into javascript thats unbuild so that I may work on it then build it again ?

what would you suggest and how do i go about updating the site ?