How to override vendor class file?

My laravel version : 5.2

I want to override tymondesigns/jwt-auth GetUserFromToken.php

\vendor\tymon\jwt-auth\src\Middleware\GetUserFromToken.php

    class GetUserFromToken extends BaseMiddleware
    {
        /**
         * Handle an incoming request.
         *
         * @param  \Illuminate\Http\Request  $request
         * @param  \Closure  $next
         * @return mixed
         */
        public function handle($request, \Closure $next)
        {
            if (! $token = $this->auth->setRequest($request)->getToken()) {

                return $this->respond('tymon.jwt.absent', 'token_not_provided', 400);
            }

For example, I want to return a custom message when token is not provided.

if (! $token = $this->auth->setRequest($request)->getToken()) {

            return my_custom_message;
        }

since I cannot make changes directly on this file, how should I create a custom class to override vendor's GetUserFromToken.php ?

Please describe required steps in detail if possible. Thanks.

via ethan17