first of all, I would like to clarify that IISc does not offer an undergraduate degree in aerospace engineering. you can apply for masters of technology in aeronautical engineering in IISc the eligibility criteria states that one must have completed the bachelor's degree in aerospace engineering after that it further states that one will have to qualify for the GATE aptitude test and then one will have to appear for the personal interview round for further information you can check the official website for the important details 

http://www.aero.iisc.ernet.in/admissions/

thanks