Yes sure you can apply for M.Tech. in Aerospace Engineering but you need to qualify for GATE in Aerospace Engineering.Few colleges don't take admission if you are not pursued b.tech. in mechanical engineering or ECE/EE. Whereas few college take direct admission if you succeed to achieve good rank.
Good Luck

Any mechanical engineer after completing his B-tech can join for Aerospace stream for his post graduation if one has good GATE rank.

But before changing the branch, you must be 100% sure about your future . Because in India there are very limited colleges such as IITs and NITs which offers you admission. So in future you may face lots of problem to join in Industry because your resume will completely change after your M-tech. Then only option remains for you after M-tech is to join for lectureship or PhD.

Some of my friends have joined for lectureship in private engineering colleges after their M-tech in Aerospace. They didt get any entry in Industry as trainee.

Thanks,
Azim