Dear Aspirant,

You can definitely change your course for doing masters. What you have to do is give GATE exam in the branch you want to do masters and if you qualify, you will be eligible to do so. 

So if you want to do masters in aerospace engineering, you have to prepare for aerospace engineering syllabus for GATE exam and clear its cutoff.

I hope the above information helps.

Good luck

Yes you can definitely do masters in whichever field you want but if the field is not relevant, you will have to study everything right from scratch and it would be difficult for you. But if you still want to do it, there's no stopping you, obviously. So all the best! Do what you like the most!