Hi Harshad

Yes, you can apply for M.tech in Aerospace engineering after B.Tech in Mechanical engineering. You can sit for the GATE exam, which offers admission into IITs and IISc, and most other government colleges. There is also TANCET exam, which offers admission into universities in Tamil Nadu. You can find the list of colleges offering M.Tech in Aerospace Engineering here

https://engineering.careers360.com/colleges/list-of-me-mtech-in-aerospace-engineering-colleges-in-india