import users from './users.js';

is similar to

const users = ...

in that you cannot assign the value afterward. It's not quite the same because the values can change, but they can only be changed from inside the module. so inside users.js

let users = [];

export const setUser(newUsers) {
  users = newUsers
}

export {users, setUser);

index.js

import {users, setUser} from './users.js'

console.log(users);
console.log(setUser(["a", "b", "c"]));

As @pilchard explained, to accomplish my task I need to change this array inside the module. That's because values that I import are read-only outside the module. Docs.

users.js

let users = ["Jack", "Mary"];

function emptyUsers() {
    users = [];
}

export { users, emptyUsers };

index.js

import { users, emptyUsers } from "./users.js"
console.log(users); // ["Jack", "Mary"]

emptyUsers(); // instead of users = [];
console.log(users); // []

users.push('Ricky'); // Also I can still change the values of the imported array
console.log(users); // ['Ricky']

try to import as different name:

import users as u from "./users.js"