Yes, you can do Masters from JNU anytime you wish if you have a graduation degree. Applying for masters in JNU does not have any agebar, you can do it as and when required.  Just prepare well for the entrance exam and you got it.

You can refer books from Arihant Publications and also you can update your G.K. by just watching news for sometime.

It's not that tough, I am sure you will be be able to clear the exam and pursue your masters.

Have a good day.