Hi Aspirant,

Visit www.nios.ac.in (NIOS official Website). Then on the left side there is a block saying information about, there click on the link stating "On Demand Examination". Then after clicking on that you will get various links related with On Demand Examination (both for Existing learner and New Learner). You can then either register yourself first or can start using as an existing user.

Hope this helps.

Good luck!!